Income Taxes - Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets—
Accounts receivable and allowance for doubtful accounts	$ 715	$ 1,627
Stock-based compensation	2,297	3,036
Accrued liabilities and expenses	19,555	23,000
Net operating loss carryforwards	6,007	5,053
Goodwill		875
Intangible assets	2,272
Other	544	759
Subtotal	31,390	34,350
Less-Valuation allowance	(3,500)	(3,184)
Total deferred tax assets	27,890	31,166
Deferred tax liabilities—
Property and equipment	(4,668)	(4,398)
Long-term contracts	(625)	(637)
Goodwill	(1,572)
Intangible assets		(737)
Other	(322)	(513)
Total deferred tax liabilities	(7,187)	(6,285)
Net deferred tax assets	20,703	24,881
Deferred income tax assets-
Deferred income tax assets	22,966	27,170
Deferred income tax liabilities-
Deferred tax liabilities	$ 2,263	$ 2,289