Summary of Significant Accounting Principles - Revenue Recognition (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Major Customer [Line Items]
Costs and estimated earnings in excess of billings on uncompleted contracts	$ 53,287		$ 53,287		$ 47,389	$ 46,006
Billings in excess of costs and estimated earnings on uncompleted contracts	25,069		25,069		35,668	33,923
Deferred income taxes	15,084		15,084		13,167	13,243
Retained earnings	68,911		68,911		62,561	62,847
Contract revenues	159,767	$ 137,420	296,610	$ 276,177
Costs of contract revenues	138,998	122,023	260,019	247,795
Gross profit	20,769	15,397	36,591	28,382
Income tax expense (benefit)	1,660	(1,624)	3,149	(2,643)
Net income (loss)	$ 2,249	$ (2,293)	$ 6,350	$ (4,101)
Basic (loss) income per share (in dollars per share)	$ 0.08	$ (0.08)	$ 0.22	$ (0.15)
Diluted (loss) income per share (in dollars per share)	$ 0.08	$ (0.08)	$ 0.22	$ (0.15)
Customer One
Revenue, Major Customer [Line Items]
Contract revenues			$ 15,600
Before ASC 606
Revenue, Major Customer [Line Items]
Costs and estimated earnings in excess of billings on uncompleted contracts	$ 53,764		53,764			46,006
Billings in excess of costs and estimated earnings on uncompleted contracts	25,818		25,818			33,923
Deferred income taxes	14,974		14,974			13,243
Retained earnings	68,749		68,749			$ 62,847
Contract revenues			295,861
Costs of contract revenues			259,542
Gross profit			36,319
Income tax expense (benefit)			3,039
Net income (loss)			$ 6,188
Basic (loss) income per share (in dollars per share)			$ 0.22
Diluted (loss) income per share (in dollars per share)			$ 0.22
Accounting Standards Update 2014-09 | ASC 606 Adjustments
Revenue, Major Customer [Line Items]
Costs and estimated earnings in excess of billings on uncompleted contracts	(477)		$ (477)		1,383
Billings in excess of costs and estimated earnings on uncompleted contracts	(749)		(749)		1,745
Deferred income taxes	110		110		(76)
Retained earnings	$ 162		162		$ (286)
Contract revenues			749
Costs of contract revenues			477
Gross profit			272
Income tax expense (benefit)			110
Net income (loss)			$ 162
Basic (loss) income per share (in dollars per share)			$ 0.00
Diluted (loss) income per share (in dollars per share)			$ 0.00