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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2019	May 05, 2019
Document and Entity Information [Abstract]
Entity Registrant Name	NATURAL GAS SERVICES GROUP INC
Entity Central Index Key	0001084991
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	true
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		13,225,077

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current Assets:
Cash and cash equivalents	$ 39,992	$ 52,628
Trade accounts receivable, net of allowance for doubtful accounts of $299 and $291, respectively	10,298	7,219
Inventory	35,499	30,974
Prepaid income taxes	3,100	3,148
Prepaid expenses and other	1,702	2,430
Total current assets	90,591	96,399
Long-term inventory, net of allowance for obsolescence of $19	3,997	3,980
Rental equipment, net of accumulated depreciation of $168,466 and $165,428, respectively	179,442	175,886
Property and equipment, net of accumulated depreciation of $11,665 and $11,556, respectively	17,134	16,587
Right of use assets - operating leases, net of accumulated amortization $36	541	0
Goodwill	10,039	10,039
Intangibles, net of accumulated amortization of $1,789 and $1,758, respectively	1,370	1,401
Other assets	1,243	1,109
Total assets	304,357	305,401
Current Liabilities:
Accounts payable	3,450	2,122
Accrued liabilities	4,341	8,743
Current operating leases	148	0
Deferred income	133	81
Total current liabilities	8,072	10,946
Line of credit	417	417
Deferred income tax liability	32,315	32,158
Long-term operating leases	393	0
Other long-term liabilities	1,764	1,699
Total liabilities	42,961	45,220
Commitments and contingencies (Note 9)
Stockholders’ Equity:
Preferred stock, 5,000 shares authorized, no shares issued or outstanding	0	0
Common stock, 30,000 shares authorized, par value $0.01; 13,133 and 13,005 shares issued and outstanding, respectively	131	130
Additional paid-in capital	108,617	107,760
Retained earnings	152,648	152,291
Total stockholders' equity	261,396	260,181
Total liabilities and stockholders' equity	$ 304,357	$ 305,401

Condensed Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current Assets:
Allowance for doubtful accounts	$ 299	$ 291
Allowance for inventory obsolescence	19	19
Noncurrent Assets:
Accumulated depreciation, rental equipment	168,466	165,428
Accumulated depreciation, property and equipment	11,665	11,556
Accumulated amortization, operating lease right of use assets	36
Accumulated amortization, intangibles	$ 1,789	$ 1,758
Stockholders’ Equity:
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in USD per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	30,000,000	30,000,000
Common stock, shares issued (in shares)	13,133,000	13,005,000
Common stock, shares outstanding (in shares)	13,133,000	13,005,000

Condensed Consolidated Income Statements - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue:
Revenue	$ 17,991	$ 14,718
Operating costs and expenses:
Cost of rentals, exclusive of depreciation stated separately below	5,885	4,704
Cost of sales, exclusive of depreciation stated separately below	3,699	2,191
Cost of service and maintenance	147	65
Selling, general and administrative expense	2,493	2,021
Depreciation and amortization	5,558	5,387
Total operating costs and expenses	17,782	14,368
Operating income	209	350
Other income (expense):
Interest expense	(4)	(3)
Other income (expense), net	305	(73)
Total other income (expense), net	301	(76)
Income before provision for income taxes	510	274
Income tax expense	153	49
Net income	$ 357	$ 225
Earnings per share:
Basic (in USD per share)	$ 0.03	$ 0.02
Diluted (in USD per share)	$ 0.03	$ 0.02
Weighted average shares outstanding:
Basic (in shares)	13,064	12,920
Diluted (in shares)	13,268	13,169
Rental
Revenue:
Revenue	$ 13,387	$ 11,471
Operating costs and expenses:
Total operating costs and expenses	5,885	4,704
Other income (expense):
Total other income (expense), net	0	0
Income before provision for income taxes	7,502	6,767
Sales
Revenue:
Revenue	4,125	2,998
Service and maintenance
Revenue:
Revenue	479	249
Operating costs and expenses:
Total operating costs and expenses	147	65
Other income (expense):
Total other income (expense), net	0	0
Income before provision for income taxes	$ 332	$ 184

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Additional Paid-In Capital	Retained Earnings
Preferred stock, shares outstanding, beginning of period (in shares) at Dec. 31, 2017		0
Common stock, shares outstanding, beginning of period (in shares) at Dec. 31, 2017			12,880,000
Balance, beginning of period at Dec. 31, 2017	$ 257,319	$ 0	$ 129	$ 105,325	$ 151,865
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options, shares (in shares)			9,000
Exercise of common stock options	157			157
Compensation expense on common stock options	66			66
Issuance of restricted stock, shares (in shares)			60,000
Issuance of restricted stock	0
Compensation expense on restricted common stock	362			362
Taxes paid related to net shares settlement of equity awards	(495)			(495)
Net income	225				225
Preferred stock, shares outstanding, end of period (in shares) at Mar. 31, 2018		0
Common stock, shares outstanding, end of period (in shares) at Mar. 31, 2018			12,949,000
Balance, end of period at Mar. 31, 2018	$ 257,634	$ 0	$ 129	105,415	152,090
Preferred stock, shares outstanding, beginning of period (in shares) at Dec. 31, 2018	0	0
Common stock, shares outstanding, beginning of period (in shares) at Dec. 31, 2018	13,005,000		13,005,000
Balance, beginning of period at Dec. 31, 2018	$ 260,181	$ 0	$ 130	107,760	152,291
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options, shares (in shares)			57,000
Exercise of common stock options	555			555
Compensation expense on common stock options	31			31
Issuance of restricted stock, shares (in shares)			71,000
Issuance of restricted stock	0
Compensation expense on restricted common stock	464		$ 1	463
Taxes paid related to net shares settlement of equity awards	(192)			(192)
Net income	$ 357				357
Preferred stock, shares outstanding, end of period (in shares) at Mar. 31, 2019	0	0
Common stock, shares outstanding, end of period (in shares) at Mar. 31, 2019	13,133,000		13,133,000
Balance, end of period at Mar. 31, 2019	$ 261,396	$ 0	$ 131	$ 108,617	$ 152,648

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 357	$ 225
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Depreciation and amortization	5,558	5,387
Deferred income taxes	157	48
Stock-based compensation	495	428
Bad debt (recovery) allowance	10	(50)
Gain on sale of assets	(22)	0
(Gain) loss on company owned life insurance	(99)	67
Changes in operating assets and liabilities:
Trade accounts receivables	(3,089)	865
Inventory	(4,468)	15
Prepaid expenses and prepaid income taxes	202	488
Accounts payable and accrued liabilities	(3,074)	(2,539)
Deferred income	52	109
Other	75	85
NET CASH (USED IN) PROVIDED BY OPERATING ACTIVITIES	(3,846)	5,128
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of rental equipment and property and equipment	(9,130)	(8,712)
Purchase of company owned life insurance	(40)	(139)
Proceeds from sale of property and equipment	11	0
Proceeds from insurance claims of property and equipment	11	0
NET CASH USED IN INVESTING ACTIVITIES	(9,148)	(8,851)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payments from other long-term liabilities, net	(5)	(9)
Proceeds from exercise of stock options	555	157
Taxes paid related to net share settlement of equity awards	(192)	(495)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	358	(347)
NET CHANGE IN CASH AND CASH EQUIVALENTS	(12,636)	(4,070)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	52,628	69,208
CASH AND CASH EQUIVALENTS AT END OF PERIOD	39,992	65,138
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Interest paid	4	3
Income taxes paid	2	0
NON-CASH TRANSACTIONS
Transfer of rental equipment components to inventory	0	40
Transfer of prepaids to rental equipment and inventory	574	$ 0
Right of use acquired through an operating lease	$ 126

Basis of Presentation and Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Basis of Presentation and Summary of Significant Accounting Policies

These notes apply to the unaudited condensed consolidated financial statements of Natural Gas Services Group, Inc. a Colorado corporation (the "Company", “NGSG”, "Natural Gas Services Group", "we" or "our"). The accompanying unaudited condensed consolidated financial statements include the accounts of the Company, its subsidiary, NGSG Properties, LLC and the rabbi trust associated with the Company's deferred compensations plan, see Note 4. All significant intercompany accounts and transactions for the periods presented have been eliminated in consolidation.

These financial statements include all adjustments, consisting of only normal recurring adjustments, which are necessary to make our financial position at March 31, 2019 and the results of our operations for the three months ended March 31, 2019 and 2018 not misleading. As permitted by the rules and regulations of the Securities and Exchange Commission (SEC), the accompanying condensed consolidated financial statements do not include all disclosures normally required by generally accepted accounting principles in the United States of America (GAAP). These financial statements should be read in conjunction with the financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2018 on file with the SEC. In our opinion, the condensed consolidated financial statements are a fair presentation of the financial position, results of operations, changes in stockholders' equity and cash flows for the periods presented.

The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results of operations to be expected for the full fiscal year ending December 31, 2019.

Revenue Recognition Policy

The Company adopted ASC 606, Revenue from Contracts with Customers ("ASC 606") on January, 1, 2018. As a result, the Company has changed its accounting policy for revenue recognition as detailed below.

The Company applied ASC 606 using the cumulative effect method. We had no significant changes in our recognition of revenue at adoption and our review of all open revenue from contracts with customers on January 1, 2018 indicated we had no adjustment to be made. If an adjustment had been needed, we would have recognized the cumulative effect of initially applying ASC 606 with an adjustment to the opening balance of equity at January 1, 2018.

Revenue is measured based on a consideration specified in a customer’s contract, excluding any sale incentives and taxes collected on behalf of third parties (i.e. sales and property taxes). We recognize revenue once a performance obligation has been satisfied and control over a product or service has transferred to the customer. Shipping and handling costs incurred are accounted for as fulfillment costs and are included in cost of revenues in our condensed consolidated income statement.

Nature of Goods and Services

The following is a description of principal activities from which the Company generates its revenue:

Rental Revenue. The Company generates revenue from renting compressors and flare systems to our customers. These contracts may also include a fee for servicing the compressor or flare during the rental contract. Our rental contracts range from six to twenty-four months, with revenue being recognized over time, in equal payments over the term of the contract. After the terms of the contract have expired, a customer may renew their contract or continue renting on a monthly basis thereafter. In accordance ASC 842 – Leases, we have applied the practical expedient ASC 842-10-15-42A, which allows the Company to combine lease and non-lease components.

Sales Revenue. The Company generates revenue by the sale of custom/fabricated compressors, flare systems and parts, as well as, exchange/rebuilding customer owned compressors and sale of used rental equipment.

Custom/fabricated compressors and flare systems - The Company designs and fabricates compressors and flares based on the customer’s specifications outlined in their contract. Though the equipment being built is customized by the customer, control under these contracts does not pass to the customer until the compressor or flare package is complete and shipped, or in accordance with a bill and hold arrangements the customer accepts title and assumes the risk and rewards of ownership. We request some of our customers to make progressive payments as the product is being built; these payments are recorded as a contract liability on the Deferred Income line on the condensed consolidated balance sheet until control has been transferred.

These contracts also may include an assurance warranty clause to guarantee the product is free from defects in material and workmanship for a set duration of time; this is a standard industry practice and is not considered a performance obligation.

From time to time, upon the customer’s written request, we recognize revenue when manufacturing is complete and the equipment is ready for shipment. At the customer’s request, we will bill the customer upon completing all performance obligations, but before shipment. The customer will formally request we ship the equipment per their direction from our manufacturing facility at a later specified date and that we segregate the equipment from our finished goods, such that they are not available to fill other orders. Per the customer’s agreement change of control is passed to the customer once the equipment is complete and ready for shipment. We have operated using bill and hold agreements with certain customers for many years, with consistent satisfactory results for both the customer and us. The credit terms on these agreements are consistent with the credit terms on all other sales. All control is shouldered by the customer and there are no exceptions to the customer’s commitment to accept and pay for the manufactured equipment. Revenues recognized related to bill and hold arrangements for the three months ended March 31, 2019 was approximately $2.7 million.

Parts - Revenue is recognized after the customer obtains control of the parts. Control is passed either by the customer taking physical possession or the parts being shipped. The amount of revenue recognized is not adjusted for expected returns, as our historical part returns have been de minimis.

Exchange or rebuilding customer owned compressors - Based on the contract, the Company will either exchange a new/rebuilt compressor for the customer’s malfunctioning compressor or rebuild the customer’s compressor. Revenue is recognized after control of the replacement compressor has transferred to the customer by physical delivery, delivery and installment or shipment of the compressor.

Used compressors or flares - From time to time, a customer may request to purchase a used compressor or flare out of our rental fleet. Revenue from the sale of rental equipment is recognized when the control has passed to the customer, when the customer has taken physical possession or the equipment has been shipped.

Service and Maintenance Revenue. The Company provides routine or call-out services on customer owned equipment. Revenue is recognized after services in the contract are rendered.

Payment terms for sales revenue and service and maintenance revenue discussed above are generally 30 to 60 days although terms for specific customers can vary. Also, the transaction prices are not subject to variable consideration constraints.

Disaggregation of Revenue

The following table shows the Company's revenue disaggregated by product or service type for the three months ended March 31, 2019 and 2018:


	Three months ended March 31,
	(in thousands)
	2019		2018
Compressors - sales	$	2,708	$	1,829
Flares - sales	435		522
Other (Parts/Rebuilds) - sales	982		647
Service and maintenance	479		249
Total revenue from contracts with customers	4,604		3,247
Add: non-ASC 606 rental revenue	13,387		11,471
Total revenue	$	17,991	$	14,718

Contract Balances

As of March 31, 2019 and December 31, 2018, we had the following receivables and deferred income from contracts with customers:


	March 31, 2019		December 31, 2018
	(in thousands)
Accounts Receivable
Accounts receivable - contracts with customers	$	3,484	$	2,250
Accounts receivable - non-ASC 606	7,113		5,260
Total Accounts Receivable	$	10,597	$	7,510
Less: Allowance for doubtful accounts	$	(299)	$	(291)
Total Accounts Receivable, net	$	10,298	$	7,219

Deferred income	$	133	$	81

The Company recognized $15,000 in revenue for the period ended March 31, 2019 that was included in deferred income at the beginning of 2019. For the period ended December 31, 2018, the Company recognized revenue of $176,000 from amounts related to sales that were included in deferred income at the beginning of 2018.

The changes in the balance of accounts receivable and deferred income were primarily due to normal timing differences between our performance and the customers’ payments.

Transaction Price Allocated to the Remaining Performance Obligations

As of March 31, 2019, the Company did not have revenue related to performance obligations under ASC 606-10-50-13.

Contract Costs

The Company applies the practical expedient in ASC 340-40-25-4 and recognizes the incremental costs of obtaining contracts as an expense when incurred if the amortization period of the assets that the Company otherwise would have recognized is one year or less. These costs are included in selling, general and administrative expense on our consolidated income statement.

Leases

On January 1, 2019, we adopted ASC 842, Leases ("ASC 842") and all the related amendments using the modified retrospective method. We recognized the cumulative effect of initially applying the new lease standard and had no adjustments to retained earnings. The comparative information has not been restated and continues to be reported under the lease accounting standard in effect for those periods. We do not expect the adoption of the new lease standard to have a material impact to our net income on an ongoing basis.

The new lease standard requires all leases to be reported on the balance sheet as right-of-use assets and lease obligations. We elected the practical expedients permitted under the transition guidance of the new standard that retained the lease classification and initial direct costs for any leases that existed prior to adoption of the standard. We did not reassess whether any contracts or land easements entered into prior to adoption are leases or contain leases.

The cumulative effect of the changes made to our consolidated balance sheet at January 1, 2019, for the adoption of ASU 2016-02, Leases, was as follows (in thousands):


	Balance at December 31, 2018	Adjustments due to ASU 2016-02	Balance at January 1, 2019
Balance Sheet
Assets
Right of use assets	—	451	451
Liabilities
Current portion of operating leases	—	126	126
Long term portion of operating leases	—	325	325

The Company, as a lessee, applies the practical expedient in ASC 842-10-15-37, and does not separate non-lease components from lease components, therefore, accounting for each separate lease component and its associated non-lease component, as a single lease component.

The Company, as a lessor, applies the practical expedient in ASC 842-10-15-42A, in leases that contain the same timing and pattern of transfer for lease and non-lease components and are deemed operating leases, the lessor is not required to separate non-lease components from lease components. Therefore, on any lease that meets these qualifications we have chosen to account for each separate lease component and its associated non-lease component, as a single lease component.

The Company applies the practical expedient in ASC 842-20-25-2, to recognize the lease payments in profit or loss on a straight-line basis over the lease term and variable lease payments in the period in which the obligation for those payments is incurred for leases with terms of 12 months or less.

Fair Value of Financial Instruments

Our financial instruments consist principally of cash and cash equivalents, accounts receivable, accounts payable, deferred compensation plan (cash portion) and our line of credit. Pursuant to ASC 820 (Accounting Standards Codification), the fair value of our cash equivalents is determined based on “Level 1” inputs, which consist of quoted prices in active markets for identical assets. We believe that the recorded values of all of our other financial instruments approximate their fair values because of their nature (variable rate debt) and relatively short maturity dates or durations.

Income Taxes

As part of the process of preparing our financial statements, we are required to estimate our federal income taxes as well as income taxes in each of the states in which we operate. This process involves us estimating our actual current tax exposure together with assessing temporary differences resulting from differing treatment of items for tax and accounting purposes. These differences result in deferred tax assets and liabilities, which are included in our consolidated balance sheet. We must then assess the likelihood that our deferred tax assets will be recovered from future taxable income and, to the extent we believe that recovery is not probable, we must establish a valuation allowance. To the extent we establish a valuation allowance or increase this allowance in a period, we must include an expense in the tax provision in the statement of operations.

Significant management judgment is required in determining our provision for income taxes, our deferred tax assets and liabilities and any valuation allowance recorded against our net deferred tax assets. We currently have no valuation allowance and fully expect to utilize all of our deferred tax assets.

ASC Topic 740 prescribes a recognition threshold and measurement attribute for the financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. In order to record any financial statement benefit, we are required to determine, based on technical merits of the position, whether it is more likely than not (a likelihood of more than 50 percent) that a tax position will be sustained upon examination, including resolution of any related appeals or litigation processes. If that step is satisfied, then we must measure the tax position to determine the amount of benefit to recognize in the financial statements. The tax position is measured at the largest amount of the benefit that has a greater than 50 percent likely of being realized upon ultimate settlement.

Our policy regarding income tax interest and penalties is to expense those items as other expense.

During the fourth quarter of 2018, the Company discovered a potential uncertain tax position attributable to the deductibility of certain executive compensation expense for federal income tax purposes aggregating approximately $168,000, $149,000 and $230,000 for the years ended December 31, 2017, 2016 and 2015, respectively. As a result, in accordance with ASC Topic 740, during the fourth quarter of 2018, the Company recorded a tax adjustment of $547,000 and accrued penalty and interest expense of $55,000 attributable to the uncertain tax position. Management of the Company determined that the effect of the potential uncertain tax position on previously reported results of operations for the years ended December 31, 2017 and 2016 was not material. The Company plans on amending its previously filed federal income tax returns and is in the process of analyzing certain offsetting deductions to reduce uncertain tax position reserve.

Reclassification of Prior Period Balances

Certain reclassifications have been made to prior period amounts, due to our adoption of ASC 842, to conform to the current-year presentation. These reclassifications had no effect on the financial statements.

Stock-Based Compensation

3 Months Ended

Mar. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation

Stock Options:

A summary of option activity under our 1998 Stock Option Plan as of December 31, 2018, and changes during the three months ended March 31, 2019 is presented below.


	Number of Stock Options	Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (years)	Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2018	283,686	$	20.46	3.58	$	434

Exercised	(56,352)	8.97		—	474
Canceled/Forfeited	(2,000)	26.66		—	—

Outstanding, March 31, 2019	225,334	$	23.28	4.18	$	26
Exercisable, March 31, 2019	214,901	$	23.04	4.00	$	26

The following table summarizes information about our stock options outstanding at March 31, 2019:


Range of Exercise Prices	Options Outstanding												Options Exercisable
		Shares		Weighted Average Remaining Contractual Life (years)			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price
$0.01-15.70	8,500		2.82		$	14.89		8,500		$	14.89
$15.71-17.81	42,000		1.32		17.54			42,000		17.54
$17.82-20.48	50,500		2.09		19.43			50,500		19.43
$20.49-33.36	124,334		6.08		27.36			113,901		27.29
	225,334		4.18		$	23.28		214,901		$	23.04

The summary of the status of our unvested stock options as of December 31, 2018 and changes during the three months ended March 31, 2019 is presented below.


Unvested stock options:	Shares	Weighted Average Grant Date Fair Value Per Share
Unvested at December 31, 2018	20,865	$	11.93

Vested	(10,432)	11.93

Unvested at March 31, 2019	10,433	$	11.93

As of March 31, 2019, there was $103,018 of unrecognized compensation cost related to unvested options. Such cost is expected to be recognized over a weighted-average period of one year. Total compensation expense for stock options was $30,779 and $66,625 for the three months ended March 31, 2019 and 2018, respectively.

Restricted Shares/Units:

In accordance with the Company's employment agreement with Stephen Taylor, the Company's Chief Executive Officer, the Compensation Committee reviewed his performance in determining the issuance of restricted common stock. Based on this review which included consideration of the Company's 2018 performance, Mr. Taylor, was awarded 131,674 restricted shares/units on March 29, 2019, which vest over three years, in equal annual installments, beginning March 29, 2020. On March 29, 2019, the Compensation Committee awarded 20,000 restricted shares/units to each of G. Larry Lawrence, our CFO, and James Hazlett, our Vice President of Technical Services. The restricted shares to Messrs. Lawrence and Hazlett vest over three years, in equal annual installments, beginning March 29, 2020. The restricted common stock awarded to Messrs. Taylor, Lawrence and Hazlett are contingent of the Company's shareholders approval and adoption of the Natural Gas Services Group, Inc. 2019 Equity Incentive Plan at the Annual Shareholder meeting on June 20, 2019. We also awarded and issued 23,136 shares of restricted common stock to the independent members of our Board of Directors as partial payment for 2019 services as directors. The restricted stock issued to our directors vests over one year, in quarterly installments, beginning March 31, 2020. Total compensation expense related to restricted awards was $463,466 and $361,630 for the three months ended March 31, 2019 and 2018, respectively. As of March 31, 2019, there was a total of $2.6 million of unrecognized compensation expense related to these shares/units which is expected to be recognized over the next two years.

Inventory

3 Months Ended

Mar. 31, 2019

Inventory Disclosure [Abstract]

Inventory

Our inventory, net of allowance for obsolescence of $19,000 at March 31, 2019 and December 31, 2018, respectively, consisted of the following amounts:


	March 31, 2019		December 31, 2018
	(in thousands)
Raw materials - current	$	29,821	$	26,152
Raw materials - long term, net	3,997		3,980
Finished Goods	1,022		1,022
Work in process	4,656		3,800
	$	39,496	$	34,954

Our long-term inventory consists of raw materials that remain viable but that the Company does not expect to sell or use within the year.

During the three months ended March 31, 2019 and 2018, there were no write-offs of obsolete inventory against the allowance for obsolescence.

Deferred Compensation Plans	3 Months Ended
Deferred Compensation Plans	Mar. 31, 2019
Postemployment Benefits [Abstract]
Deferred Compensation Plans	Deferred Compensation Plans The Company has a non-qualified deferred compensation plan for executive officers, directors and certain eligible employees. The assets of the deferred compensation plan are held in a rabbi trust and are subject to additional risk of loss in the event of bankruptcy or insolvency of the Company. The plan allows for deferral of up to 90% of a participant’s base salary, bonus, commissions, director fees and restricted stock unit awards. A Company owned life insurance policy held in a rabbi trust is utilized as a source of funding for the plan. The cash surrender value of the life insurance policy is $1.2 million and $966,000 as of March 31, 2019 and 2018, respectively. For the three months ending March 31, 2019, we reported in other income (expense) in the consolidated income statement a gain related to the policy of approximately $99,000 and for the same period in 2018, a loss of approximately $67,000. For deferrals of base salary, bonus, commissions and director fees, settlement payments are made to participants in cash, either in a lump sum or in periodic installments. The obligation to pay the deferred compensation and the deferred director fees is adjusted to reflect the positive or negative performance of investment measurement options selected by each participant and was $1.2 million and $992,000 as of March 31, 2019 and 2018, respectively. The deferred obligation is included in other long-term liabilities in the condensed consolidated balance sheet. For deferrals of restricted stock units, the plan does not allow for diversification, therefore, distributions are paid in shares of common stock and the obligation is carried at grant value. As of March 31, 2019 and 2018, respectively, 63,062 and 104,590 unvested restricted stock units have been deferred of which 78,771 and 32,037 units have been released and issued to the deferred compensation plan with a value of $1.7 million and $816,373, respectively.

Credit Facility	3 Months Ended
Credit Facility	Mar. 31, 2019
Debt Disclosure [Abstract]
Credit Facility	Credit Facility We have a senior secured revolving credit agreement the ("Amended Credit Agreement") with JP Morgan Chase Bank, N.A (the "Lender") with an aggregate commitment of $30 million, subject to collateral availability. We also have a right to request from the Lender, on an uncommitted basis, an increase of up to $20 million on the aggregate commitment (which could potentially increase the commitment amount to $50 million). On August 31, 2017, we amended and renewed the Amended Credit Agreement, extending the maturity date to December 31, 2020. No other material revisions were made to the credit facility. Borrowing Base. At any time before the maturity of the Amended Credit Agreement, we may draw, repay and re-borrow amounts available under the borrowing base up to the maximum aggregate availability discussed above. Generally, the borrowing base equals the sum of (a) 80% of our eligible accounts receivable plus (b) 50% of the book value of our eligible general inventory (not to exceed 50% of the commitment amount at the time) plus (c) 75% of the book value of our eligible equipment inventory. The Lender may adjust the borrowing base components if material deviations in the collateral are discovered in future audits of the collateral. We had $29.5 million borrowing base availability at March 31, 2019 under the terms of our Amended Credit Agreement. Interest and Fees. Under the terms of the Amended Credit Agreement, we have the option of selecting the applicable variable rate for each revolving loan, or portion thereof, of either (a) LIBOR multiplied by the Statutory Reserve Rate (as defined in the Amended Credit Agreement), with respect to this rate, for Eurocurrency funding, plus the Applicable Margin (“LIBOR-based”), or (b) CB Floating Rate, which is the Lender's Prime Rate less the Applicable Margin; provided, however, that no more than three LIBOR-based borrowings under the agreement may be outstanding at any one time. For purposes of the LIBOR-based interest rate, the Applicable Margin is 1.50%. For purposes of the CB Floating Rate, the Applicable Margin is 1.50%. For the three month period ended March 31, 2019, our weighted average interest rate was 3.75%. Accrued interest is payable monthly on outstanding principal amounts, provided that accrued interest on LIBOR-based loans is payable at the end of each interest period, but in no event less frequently than quarterly. In addition, fees and expenses are payable in connection with our requests for letters of credit (generally equal to the Applicable Margin for LIBOR-related borrowings multiplied by the face amount of the requested letter of credit) and administrative and legal costs. Maturity. The maturity date of the Amended Credit Agreement is December 31, 2020, at which time all amounts borrowed under the agreement will be due and outstanding letters of credit must be cash collateralized. The agreement may be terminated early upon our request or the occurrence of an event of default. Security. The obligations under the Amended Credit Agreement are secured by a first priority lien on all of our inventory and accounts and leases receivables, along with a first priority lien on a variable number of our leased compressor equipment the book value of which must be maintained at a minimum of 2.00 to 1.00 commitment coverage ratio (such ratio being equal to (i) the amount of the borrowing base as of such date to (ii) the amount of the commitment as of such date). Covenants. The Amended Credit Agreement contains customary representations and warranties, as well as covenants which, among other things, limit our ability to incur additional indebtedness and liens; enter into transactions with affiliates; make acquisitions in excess of certain amounts; pay dividends; redeem or repurchase capital stock or senior notes; make investments or loans; make negative pledges; consolidate, merge or effect asset sales; or change the nature of our business. In addition, we also have certain financial covenants that require us to maintain on a consolidated basis a leverage ratio less than or equal to 2.50 to 1.00 as of the last day of each fiscal quarter. Events of Default and Acceleration. The Amended Credit Agreement contains customary events of default for credit facilities of this size and type, and includes, without limitation, payment defaults; defaults in performance of covenants or other agreements contained in the loan documents; inaccuracies in representations and warranties; certain defaults, termination events or similar events; certain defaults with respect to any other Company indebtedness in excess of $50,000; certain bankruptcy or insolvency events; the rendering of certain judgments in excess of $150,000; certain ERISA events; certain change in control events and the defectiveness of any liens under the secured revolving credit facility. Obligations under the Amended Credit Agreement may be accelerated upon the occurrence of an event of default. As of March 31, 2019, we were in compliance with all covenants in our Amended Credit Agreement. A default under our Credit Agreement could trigger the acceleration of our bank debt so that it is immediately due and payable. Such default would likely limit our ability to access other credit. At March 31, 2019 and December 31, 2018, our outstanding balance on the line of credit was $417,000.

Leases

3 Months Ended

Mar. 31, 2019

Leases [Abstract]

Leases

The Company adopted the new lease standard on January 1, 2019 using the modified retrospective transition method. Prior periods were not retrospectively adjusted and continue to be reported under the accounting standards in effect for those periods. The Company elected the package of practical expedients permitted under the transition guidance within the new lease standard, which among other things, allowed the company to continue to account for existing leases based on the historical lease classification. The Company also elected the practical expedients to exclude right-of-use ("ROU") assets and lease liabilities for leases with an initial term of 12 months or less from the balance sheet, and to combine lease and non-lease components for property leases, which primarily relate to ancillary expenses such as common area maintenance charges and management fees.

The Company determines if an arrangement is a lease at inception by assessing whether it conveys the right to control the use of an identified asset for a period of time in exchange for consideration. The Company’s leases are primarily related to property leases for its field offices. The Company's leases have remaining lease terms of one to 10 years. Renewal and termination options are included in the lease term when it is reasonably certain that the Company will exercise the option.

The Company's lease agreements do not contain any contingent rental payments, material residual guarantees or material restrictive covenants.

ROU assets and lease liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. As substantially all of the Company's leases do not provide an implicit rate, the Company uses its incremental borrowing rate to determine the present value of lease payments. Based on the present value of lease payments for the Company's existing leases, the Company recorded net lease assets and lease liabilities of approximately $451,000, respectively, upon adoption. The Company had no finance leases. The new lease standard did not materially impact the Company's consolidated statements of operations and had no impact on the Company's consolidated statements of cash flows.

The impact of the new lease standard on the March 31, 2019 consolidated balance sheet was as follows:


	Classification on the Condensed Consolidated Balance Sheet	March 31, 2019
		(in thousands, except years)
Operating lease assets	Right of use assets-operating leases	$	541

Current lease liabilities	Current operating leases	148
Noncurrent lease liabilities	Long-term operating leases	393
Total lease liabilities, net		541

Weighted average remaining lease term in years		3.1
Implicit Rate		3.8		%

Operating lease costs are recognized on a straight-line basis over the lease term. Total operating lease costs for the three months ended March 31, 2019 was approximately $193,000, which included approximately $146,000 related to short-term lease costs.


	March 31, 2019
	(in thousands)
Cash paid for amounts included in the measurement of lease liabilities
Operating lease cost⁽¹⁾	$	48

(1) Lease costs are classified on the condensed consolidated statements of income in cost of sales, cost of compressors and selling, general and administrative expenses.

The following table shows the future maturities of lease liabilities:


Years Ending December 31,	Lease Liabilities
	(in thousands)
2019 (excluding the three months ended March 31, 2019)	$	134
2020	$	116
2021	$	95
2022	$	43
2023	$	35
Thereafter	$	190
Total lease payments	$	613
Less: Imputed interest	$	72
Total	$	541

As previously disclosed on Form 10-K and under the previous lease standard (Topic 840), future minimum obligations under lease commitments in effect at December 31, 2018 as follows:


			Operating Leases
			(in thousands)
2019			298
2020			118
2021			97
2022			44
2023			35
Thereafter			15
Total			607

Earnings per Share

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Earnings per Share

The following table reconciles the numerators and denominators of the basic and diluted earnings per share computation (in thousands, except per share data):


	Three months ended
	March 31,
	2019		2018
Numerator:
Net income	$	357	$	225
Denominator for basic net income per common share:
Weighted average common shares outstanding	13,064		12,920
Denominator for diluted net income per share:
Weighted average common shares outstanding	13,064		12,920
Dilutive effect of stock options and restricted stock	204		249
Diluted weighted average shares	13,268		13,169
Earnings per common share:
Basic	$	0.03	$	0.02
Diluted	$	0.03	$	0.02

In the three months ended March 31, 2019, options to purchase 174,834 shares of common stock with exercise prices ranging from $18.75 to $33.36 were not included in the computation of diluted income per share, due to their antidilutive effect.

In the three months ended March 31, 2018, options to purchase 83,917 shares of common stock with exercise prices ranging from $28.15 to $33.36 were not included in the computation of diluted income per share, due to their antidilutive effect.

Segment Information

3 Months Ended

Mar. 31, 2019

Segment Reporting [Abstract]

Segment Information

ASC 280-10-50, “Operating Segments", defines the characteristics of an operating segment as: a) being engaged in business activity from which it may earn revenue and incur expenses, b) being reviewed by the Company's chief operating decision maker (CODM) for decisions about resources to be allocated and assess its performance and c) having discrete financial information. Although we look at our products to analyze the nature of our revenue, other financial information, such as certain costs and expenses, net income and EBITDA are not captured or analyzed by these categories. Our CODM does not

make resource allocation decisions or access the performance of the business based on these categories, but rather in the aggregate. Based on this, management believes that it operates in one business segment.

In their analysis of product lines as potential operating segments, management also considered ASC 280-10-50-11, “Aggregation Criteria”, which allows for the aggregation of operating segments if the segments have similar economic characteristics and if the segments are similar in each of the following areas:

•The nature of the products and services;

•The nature of the production processes;

•The type or class of customer for their products and services;

•The methods used to distribute their products or provide their services; and

•The nature of the regulatory environment, if applicable.

We are engaged in the business of designing and manufacturing compressors and flares. Our compressors and flares are sold and rented to our customers. In addition, we provide service and maintenance on compressors in our fleet and to third parties. These business activities are similar in all geographic areas. Our manufacturing process is essentially the same for the entire Company and is performed in-house at our facilities in Midland, Texas and Tulsa, Oklahoma. Our customers primarily consist of entities in the business of producing natural gas and crude oil. The maintenance and service of our products is consistent across the entire Company and is performed via an internal fleet of vehicles. The regulatory environment is similar in every jurisdiction in that the most impacting regulations and practices are the result of federal energy policy. In addition, the economic characteristics of each customer arrangement are similar in that we maintain policies at the corporate level.


For the three months ended March 31, 2019 (in thousands):
	Rental		Sales		Service & Maintenance		Corporate		Total
Revenue	$	13,387	$	4,125	$	479	$	—	$	17,991
Operating costs and corporate expenses	5,885		3,699		147		8,051		17,782
Total other income, net	—		—		—		301		301
Income before provision for income taxes	$	7,502	$	426	$	332	$	(7,750)	$	510


For the three months ended March 31, 2018 (in thousands):
	Rental		Sales		Service & Maintenance		Corporate		Total
Revenue	$	11,471	$	2,998	$	249	$	—	$	14,718
Operating costs and corporate expenses	4,704		2,191		65		7,408		14,368
Total other expense, net	—		—		—		(76)		(76)
Income before provision for income taxes	$	6,767	$	807	$	184	$	(7,484)	$	274

Commitments and Contingencies	3 Months Ended
Commitments and Contingencies	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and ContingenciesFrom time to time, we are a party to various legal proceedings in the ordinary course of our business. While management is unable to predict the ultimate outcome of these actions, it believes that any ultimate liability arising from these actions will not have a material adverse effect on our financial position, results of operations or cash flow. We are not currently a party to any material legal proceedings, and we are not aware of any threatened material litigation. The Company also has a remaining contractual obligation related to the construction of a new corporate office of approximately $3.7 million, to be financed by cash on hand. Construction of a new office began in late 2017 and is expected to be completed in mid-2019.

Related Party	3 Months Ended
Related Party	Mar. 31, 2019
Related Party Transactions [Abstract]
Related Party	Related Party In 2016, we entered into a joint venture partnership, N-G, LLC (‘N-G”), with Genis Holdings, LLC (“Genis”) to explore new technologies for wellhead compression. NGS and Genis both share 50% ownership of N-G. In 2018, we ordered some compressor packages from Genis, totaling $1.0 million. The compressors were completed by March 31, 2019. As of March 31, 2019, we have paid the units in full which has been reclassified from prepaids and is included in rental equipment on the consolidated balance sheet.

Basis of Presentation and Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Basis of Accounting

The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results of operations to be expected for the full fiscal year ending December 31, 2019.

Revenue Recognition

Revenue Recognition Policy

Nature of Goods and Services

The following is a description of principal activities from which the Company generates its revenue:

Service and Maintenance Revenue. The Company provides routine or call-out services on customer owned equipment. Revenue is recognized after services in the contract are rendered.

Contract Costs

Contract Costs The Company applies the practical expedient in ASC 340-40-25-4 and recognizes the incremental costs of obtaining contracts as an expense when incurred if the amortization period of the assets that the Company otherwise would have recognized is one year or less. These costs are included in selling, general and administrative expense on our consolidated income statement.

Leases

The cumulative effect of the changes made to our consolidated balance sheet at January 1, 2019, for the adoption of ASU 2016-02, Leases, was as follows (in thousands):


	Balance at December 31, 2018	Adjustments due to ASU 2016-02	Balance at January 1, 2019
Balance Sheet
Assets
Right of use assets	—	451	451
Liabilities
Current portion of operating leases	—	126	126
Long term portion of operating leases	—	325	325

Fair Value of Financial Instruments

Income Taxes

Our policy regarding income tax interest and penalties is to expense those items as other expense.

Reclassification of Prior Period Balances

Segment Information

•The nature of the products and services;

•The nature of the production processes;

•The type or class of customer for their products and services;

•The methods used to distribute their products or provide their services; and

•The nature of the regulatory environment, if applicable.

- Definition

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.

+ References + Details

Basis of Presentation and Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Disaggregation of Revenue

The following table shows the Company's revenue disaggregated by product or service type for the three months ended March 31, 2019 and 2018:


	Three months ended March 31,
	(in thousands)
	2019		2018
Compressors - sales	$	2,708	$	1,829
Flares - sales	435		522
Other (Parts/Rebuilds) - sales	982		647
Service and maintenance	479		249
Total revenue from contracts with customers	4,604		3,247
Add: non-ASC 606 rental revenue	13,387		11,471
Total revenue	$	17,991	$	14,718

Schedule of Contract with Customer, Asset and Liability

As of March 31, 2019 and December 31, 2018, we had the following receivables and deferred income from contracts with customers:


	March 31, 2019		December 31, 2018
	(in thousands)
Accounts Receivable
Accounts receivable - contracts with customers	$	3,484	$	2,250
Accounts receivable - non-ASC 606	7,113		5,260
Total Accounts Receivable	$	10,597	$	7,510
Less: Allowance for doubtful accounts	$	(299)	$	(291)
Total Accounts Receivable, net	$	10,298	$	7,219

Deferred income	$	133	$	81

Cumulative Effect of Changes from Adoption of ASU

The cumulative effect of the changes made to our consolidated balance sheet at January 1, 2019, for the adoption of ASU 2016-02, Leases, was as follows (in thousands):


	Balance at December 31, 2018	Adjustments due to ASU 2016-02	Balance at January 1, 2019
Balance Sheet
Assets
Right of use assets	—	451	451
Liabilities
Current portion of operating leases	—	126	126
Long term portion of operating leases	—	325	325

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Option Activity

A summary of option activity under our 1998 Stock Option Plan as of December 31, 2018, and changes during the three months ended March 31, 2019 is presented below.


	Number of Stock Options	Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (years)	Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2018	283,686	$	20.46	3.58	$	434

Exercised	(56,352)	8.97		—	474
Canceled/Forfeited	(2,000)	26.66		—	—

Outstanding, March 31, 2019	225,334	$	23.28	4.18	$	26
Exercisable, March 31, 2019	214,901	$	23.04	4.00	$	26

Summary of Stock Options Outstanding

The following table summarizes information about our stock options outstanding at March 31, 2019:


Range of Exercise Prices	Options Outstanding												Options Exercisable
		Shares		Weighted Average Remaining Contractual Life (years)			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price
$0.01-15.70	8,500		2.82		$	14.89		8,500		$	14.89
$15.71-17.81	42,000		1.32		17.54			42,000		17.54
$17.82-20.48	50,500		2.09		19.43			50,500		19.43
$20.49-33.36	124,334		6.08		27.36			113,901		27.29
	225,334		4.18		$	23.28		214,901		$	23.04

Status of Unvested Stock Options

The summary of the status of our unvested stock options as of December 31, 2018 and changes during the three months ended March 31, 2019 is presented below.


Unvested stock options:	Shares	Weighted Average Grant Date Fair Value Per Share
Unvested at December 31, 2018	20,865	$	11.93

Vested	(10,432)	11.93

Unvested at March 31, 2019	10,433	$	11.93

Inventory (Tables)

3 Months Ended

Mar. 31, 2019

Inventory Disclosure [Abstract]

Schedule of Inventory

Our inventory, net of allowance for obsolescence of $19,000 at March 31, 2019 and December 31, 2018, respectively, consisted of the following amounts:


	March 31, 2019		December 31, 2018
	(in thousands)
Raw materials - current	$	29,821	$	26,152
Raw materials - long term, net	3,997		3,980
Finished Goods	1,022		1,022
Work in process	4,656		3,800
	$	39,496	$	34,954

Leases (Tables)

3 Months Ended

Mar. 31, 2019

Leases [Abstract]

Schedule of Balance Sheet Impact

The impact of the new lease standard on the March 31, 2019 consolidated balance sheet was as follows:


	Classification on the Condensed Consolidated Balance Sheet	March 31, 2019
		(in thousands, except years)
Operating lease assets	Right of use assets-operating leases	$	541

Current lease liabilities	Current operating leases	148
Noncurrent lease liabilities	Long-term operating leases	393
Total lease liabilities, net		541

Weighted average remaining lease term in years		3.1
Implicit Rate		3.8		%

Schedule of Cash Flow Impact


	March 31, 2019
	(in thousands)
Cash paid for amounts included in the measurement of lease liabilities
Operating lease cost⁽¹⁾	$	48

(1) Lease costs are classified on the condensed consolidated statements of income in cost of sales, cost of compressors and selling, general and administrative expenses.

Schedule of Future Maturities of Lease Liabilities

The following table shows the future maturities of lease liabilities:


Years Ending December 31,	Lease Liabilities
	(in thousands)
2019 (excluding the three months ended March 31, 2019)	$	134
2020	$	116
2021	$	95
2022	$	43
2023	$	35
Thereafter	$	190
Total lease payments	$	613
Less: Imputed interest	$	72
Total	$	541

Schedule of Future Minimum Obligations Under Lease Commitments

As previously disclosed on Form 10-K and under the previous lease standard (Topic 840), future minimum obligations under lease commitments in effect at December 31, 2018 as follows:


			Operating Leases
			(in thousands)
2019			298
2020			118
2021			97
2022			44
2023			35
Thereafter			15
Total			607

Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2019

Earnings Per Share [Abstract]

Schedule of Earnings Per Share, Basic and Diluted

The following table reconciles the numerators and denominators of the basic and diluted earnings per share computation (in thousands, except per share data):


	Three months ended
	March 31,
	2019		2018
Numerator:
Net income	$	357	$	225
Denominator for basic net income per common share:
Weighted average common shares outstanding	13,064		12,920
Denominator for diluted net income per share:
Weighted average common shares outstanding	13,064		12,920
Dilutive effect of stock options and restricted stock	204		249
Diluted weighted average shares	13,268		13,169
Earnings per common share:
Basic	$	0.03	$	0.02
Diluted	$	0.03	$	0.02

Segment Information (Tables)

3 Months Ended

Mar. 31, 2019

Segment Reporting [Abstract]

Schedule of Segment Reporting Information


For the three months ended March 31, 2019 (in thousands):
	Rental		Sales		Service & Maintenance		Corporate		Total
Revenue	$	13,387	$	4,125	$	479	$	—	$	17,991
Operating costs and corporate expenses	5,885		3,699		147		8,051		17,782
Total other income, net	—		—		—		301		301
Income before provision for income taxes	$	7,502	$	426	$	332	$	(7,750)	$	510


For the three months ended March 31, 2018 (in thousands):
	Rental		Sales		Service & Maintenance		Corporate		Total
Revenue	$	11,471	$	2,998	$	249	$	—	$	14,718
Operating costs and corporate expenses	4,704		2,191		65		7,408		14,368
Total other expense, net	—		—		—		(76)		(76)
Income before provision for income taxes	$	6,767	$	807	$	184	$	(7,484)	$	274

Basis of Presentation and Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Revenue from External Customer [Line Items]
Revenue	$ 17,991		$ 14,718
Additions to tax positions of prior years		$ 547
Income tax penalties and interest accrued		55
Bill and Hold Arrangement
Revenue from External Customer [Line Items]
Revenue	$ 2,700
Minimum
Revenue from External Customer [Line Items]
Amortization period of capitalized contract costs	1 year
Tax Year 2017
Revenue from External Customer [Line Items]
Additions to tax positions of prior years		168
Tax Year 2016
Revenue from External Customer [Line Items]
Additions to tax positions of prior years		149
Tax Year 2015
Revenue from External Customer [Line Items]
Additions to tax positions of prior years		$ 230

Basis of Presentation and Summary of Significant Accounting Policies (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	$ 4,604	$ 3,247
Add: non-ASC 606 rental revenue	13,387	11,471
Revenue	17,991	14,718
Compressors - sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	2,708	1,829
Flares - sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	435	522
Other (Parts/Rebuilds) - sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	982	647
Service and maintenance
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	479	249
Revenue	$ 479	$ 249

Basis of Presentation and Summary of Significant Accounting Policies (Contract Balances) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Accounts Receivable
Accounts receivable - contracts with customers	$ 3,484	$ 2,250
Accounts receivable - non-ASC 606	7,113	5,260
Total Accounts Receivable	10,597	7,510
Less: Allowance for doubtful accounts	(299)	(291)
Total Accounts Receivable, net	10,298	7,219
Deferred income	133	81
Revenue recognized	$ 15	$ 176

Basis of Presentation and Summary of Significant Accounting Policies (Cumulative Effect of Changes from Adoption of ASU) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Assets
Right of use assets	$ 541	$ 451	$ 0
Liabilities
Current portion of operating leases	148	126	0
Long term portion of operating leases	$ 393	325	$ 0
Accounting Standards Update 2016-02
Assets
Right of use assets		451
Liabilities
Current portion of operating leases		126
Long term portion of operating leases		$ 325

Stock-Based Compensation (Stock Option Activity) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Number ofStock Options
Outstanding, beginning of period (in shares)	283,686
Exercised (in shares)	(56,352)
Canceled/Forfeited (in shares)	(2,000)
Outstanding, end of period (in shares)	225,334	283,686
Exercisable (in shares)	214,901
Weighted AverageExercise Price
Outstanding, beginning of period (in USD per share)	$ 20.46
Exercised (in USD per share)	8.97
Canceled/Forfeited (in USD per share)	26.66
Outstanding, end of period (in USD per share)	23.28	$ 20.46
Exercisable (in USD per share)	$ 23.04
Weighted Average Remaining Contractual Life (years)
Outstanding	4 years 2 months 4 days	3 years 6 months 29 days
Exercisable	4 years
Aggregate Intrinsic Value (in thousands)
Outstanding, beginning of period	$ 434
Exercised	474
Canceled/Forfeited	0
Outstanding, end of period	26	$ 434
Exercisable	$ 26

Stock-Based Compensation (Stock Options Outstanding) (Details)	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, shares (in shares) \| shares	225,334
Options outstanding, weighted average remaining contractual life (years)	4 years 2 months 4 days
Options outstanding, weighted average exercise price (in USD per share)	$ 23.28
Options exercisable, shares (in shares) \| shares	214,901
Options exercisable, weighted average exercise price (in USD per share)	$ 23.04
$0.01-15.70
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower limit (in USD per share)	0.01
Range of exercise prices, upper limit (in USD per share)	$ 15.70
Options outstanding, shares (in shares) \| shares	8,500
Options outstanding, weighted average remaining contractual life (years)	2 years 9 months 25 days
Options outstanding, weighted average exercise price (in USD per share)	$ 14.89
Options exercisable, shares (in shares) \| shares	8,500
Options exercisable, weighted average exercise price (in USD per share)	$ 14.89
$15.71-17.81
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower limit (in USD per share)	15.71
Range of exercise prices, upper limit (in USD per share)	$ 17.81
Options outstanding, shares (in shares) \| shares	42,000
Options outstanding, weighted average remaining contractual life (years)	1 year 3 months 25 days
Options outstanding, weighted average exercise price (in USD per share)	$ 17.54
Options exercisable, shares (in shares) \| shares	42,000
Options exercisable, weighted average exercise price (in USD per share)	$ 17.54
$17.82-20.48
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower limit (in USD per share)	17.82
Range of exercise prices, upper limit (in USD per share)	$ 20.48
Options outstanding, shares (in shares) \| shares	50,500
Options outstanding, weighted average remaining contractual life (years)	2 years 1 month 2 days
Options outstanding, weighted average exercise price (in USD per share)	$ 19.43
Options exercisable, shares (in shares) \| shares	50,500
Options exercisable, weighted average exercise price (in USD per share)	$ 19.43
$20.49-33.36
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower limit (in USD per share)	20.49
Range of exercise prices, upper limit (in USD per share)	$ 33.36
Options outstanding, shares (in shares) \| shares	124,334
Options outstanding, weighted average remaining contractual life (years)	6 years 29 days
Options outstanding, weighted average exercise price (in USD per share)	$ 27.36
Options exercisable, shares (in shares) \| shares	113,901
Options exercisable, weighted average exercise price (in USD per share)	$ 27.29

Stock-Based Compensation (Summary of Unvested Stock Options) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Shares
Unvested, beginning of period (in shares)	20,865
Vested (in shares)	(10,432)
Unvested, end of period (in shares)	10,433
Weighted Average Grant Date Fair Value Per Share
Unvested, weighted average grant date fair value, beginning of period (in USD per share)	$ 11.93
Vested, weighted average grant date fair value (in USD per share)	11.93
Unvested, weighted average grant date fair value, end of period (in USD per share)	$ 11.93
Stock Options
Weighted Average Grant Date Fair Value Per Share
Unrecognized compensation cost related to unvested options	$ 103,018
Unrecognized compensation cost related to awards, weighted average period for recognition	1 year
Share-based compensation expense	$ 30,779	$ 66,625

Stock-Based Compensation (Restricted Stock) (Details) - Restricted Stock - USD ($)		3 Months Ended
	Mar. 29, 2019	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 463,466	$ 361,630
Total unrecognized compensation expense		$ 2,600,000
Unrecognized compensation cost related to awards, vesting period		2 years
Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period (in shares)	131,674
Award vesting period	3 years
Chief Financial Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period (in shares)	20,000
Award vesting period	3 years
Vice President of Technical Services
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period (in shares)	20,000
Award vesting period	3 years
Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period (in shares)		23,136
Award vesting period		1 year

Inventory (Details) - USD ($)	3 Months Ended
Inventory (Details) - USD ($)	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Inventory Disclosure [Abstract]
Allowance for inventory obsolescence	$ 19,000		$ 19,000
Raw materials - current	29,821,000		26,152,000
Raw materials - long term, net	3,997,000		3,980,000
Finished Goods	1,022,000		1,022,000
Work in process	4,656,000		3,800,000
Inventory, net	39,496,000		$ 34,954,000
Write-offs of obsolete inventory against allowance	$ 0	$ 0

Deferred Compensation Plans (Details) - USD ($)	3 Months Ended
Deferred Compensation Plans (Details) - USD ($)	Mar. 31, 2019	Mar. 31, 2018
Postemployment Benefits [Abstract]
Participant's maximum compensation deferral percentage	90.00%
Company owned life insurance	$ 1,200,000	$ 966,000
Gain (loss) on company owned life insurance	99,000	(67,000)
Deferred compensation obligation	$ 1,200,000	$ 992,000
Deferred restricted stock shares (in shares)	63,062	104,590
Deferred compensation arrangement with individual, shares issued (in shares)	78,771	32,037
Deferred compensation arrangement, fair value of shares issued	$ 1,700,000	$ 816,373

Credit Facility (Details) - Revolving Credit Facility	3 Months Ended
Credit Facility (Details) - Revolving Credit Facility	Mar. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Aggregate credit agreement commitment	$ 30,000,000
Potential increase in borrowing capacity	20,000,000
Potential maximum borrowing capacity	$ 50,000,000
Borrowing base, component, % of eligible accounts receivable	80.00%
Borrowing base, component, % of eligible inventory	50.00%
Borrowing base, allowable share of total commitment amount attributable to inventory component	50.00%
Borrowing base, component, % of eligible equipment inventory	75.00%
Borrowing base amount available	$ 29,500,000
Weighted average interest rate	3.75%
Default trigger, certain defaults of other company indebtedness, amount	$ 50,000
Default trigger, rendering of certain judgments, amount	150,000
Line of credit balance	$ 417,000	$ 417,000
LIBOR Rate
Line of Credit Facility [Line Items]
Reference rate, number of allowable LIBOR-based borrowings outstanding \| loan	3
Variable rate, applicable margin	1.50%
CB Floating Rate
Line of Credit Facility [Line Items]
Variable rate, applicable margin	1.50%
Minimum
Line of Credit Facility [Line Items]
Minimum commitment coverage ratio allowed	2.00
Maximum
Line of Credit Facility [Line Items]
Maximum leverage ratio allowed	2.50

Leases (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
Leases (Narrative) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Right of use assets	$ 541	$ 451	$ 0
Operating lease, liability	541
Lease costs	193
Short-term lease cost	$ 146
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Right of use assets		451
Operating lease, liability		$ 451
Minimum
Lessee, Lease, Description [Line Items]
Remaining lease term	1 year
Maximum
Lessee, Lease, Description [Line Items]
Remaining lease term	10 years

Leases (Balance Sheet Impact) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Leases [Abstract]
Right of use assets	$ 541	$ 451	$ 0
Current operating leases	148	126	0
Long-term operating leases	393	$ 325	$ 0
Total lease liabilities, net	$ 541
Weighted average remaining lease term in years	3 years 1 month 6 days
Implicit Rate	3.80%

Leases (Cash Flow Impact) (Details) $ in Thousands	3 Months Ended
Leases (Cash Flow Impact) (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Leases [Abstract]
Operating lease cost	$ 48

Leases (Future Maturities of Lease Liabilities) (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Leases [Abstract]
2019 (excluding the three months ended March 31, 2019)	$ 134
2020	116
2021	95
2022	43
2023	35
Thereafter	190
Total lease payments	613
Less: Imputed interest	72
Operating lease, liability	$ 541

Leases (Future Minimum Obligations Under Lease Commitments) (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Leases [Abstract]
2019	$ 298
2020	118
2021	97
2022	44
2023	35
Thereafter	15
Total	$ 607

Earnings per Share (Basic and Diluted) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income	$ 357	$ 225
Denominator for basic net income per common share:
Weighted average common shares outstanding (in shares)	13,064,000	12,920,000
Denominator for diluted net income per share:
Dilutive effect of stock options and restricted stock (in shares)	204,000	249,000
Diluted weighted average shares (in shares)	13,268,000	13,169,000
Earnings per common share:
Basic (in USD per share)	$ 0.03	$ 0.02
Diluted (in USD per share)	$ 0.03	$ 0.02
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of shares not included in the computation of dilutive income per share (in shares)	174,834	83,917
Antidilutive Effect
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Exercise price of shares not included in the computation of dilutive income per share, lower limit (in USD per share)	$ 18.75	$ 28.15
Exercise price of shares not included in the computation of dilutive income per share, upper limit (in USD per share)	$ 33.36	$ 33.36

Segment Information (Details) $ in Thousands	3 Months Ended
Segment Information (Details) $ in Thousands	Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments \| segment	1
Revenue	$ 17,991	$ 14,718
Operating costs and corporate expenses	17,782	14,368
Total other income (expense), net	301	(76)
Income before provision for income taxes	510	274
Corporate
Segment Reporting Information [Line Items]
Revenue	0	0
Operating costs and corporate expenses	8,051	7,408
Total other income (expense), net	301	(76)
Income before provision for income taxes	(7,750)	(7,484)
Rental
Segment Reporting Information [Line Items]
Revenue	13,387	11,471
Operating costs and corporate expenses	5,885	4,704
Total other income (expense), net	0	0
Income before provision for income taxes	7,502	6,767
Sales
Segment Reporting Information [Line Items]
Revenue	4,125	2,998
Operating costs and corporate expenses	3,699	2,191
Total other income (expense), net	0	0
Income before provision for income taxes	426	807
Service and maintenance
Segment Reporting Information [Line Items]
Revenue	479	249
Operating costs and corporate expenses	147	65
Total other income (expense), net	0	0
Income before provision for income taxes	$ 332	$ 184

Commitments and Contingencies (Details) $ in Millions	Mar. 31, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Contractual obligation	$ 3.7

Related Party (Narrative) (Details) - Corporate Joint Venture - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2016
Related Party Transaction [Line Items]
Purchases from joint venture	$ 1.0
N-G, LLC
Related Party Transaction [Line Items]
Percent ownership		50.00%
Genis Holdings, LLC \| N-G, LLC
Related Party Transaction [Line Items]
Percent ownership		50.00%