Summary of Significant Accounting Principles - Revenue Recognition (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Major Customer [Line Items]
Costs and estimated earnings in excess of billings on uncompleted contracts	$ 42,706		$ 47,389	$ 46,006
Billings in excess of costs and estimated earnings on uncompleted contracts	35,769		35,668	33,923
Deferred income taxes	14,361		13,167	13,243
Retained earnings	66,662		62,561	62,847
Contract revenues	136,843	$ 138,757
Costs of contract revenues	121,021	125,772
Gross profit	15,822	12,985
Income tax expense (benefit)	1,489	(1,019)
Net income (loss)	$ 4,101	$ (1,808)
Basic (loss) income per share (in dollars per share)	$ 0.15	$ (0.07)
Diluted (loss) income per share (in dollars per share)	$ 0.14	$ (0.07)
Customer One
Revenue, Major Customer [Line Items]
Contract revenues	$ 12,300
Before ASC 606
Revenue, Major Customer [Line Items]
Costs and estimated earnings in excess of billings on uncompleted contracts	42,959			46,006
Billings in excess of costs and estimated earnings on uncompleted contracts	36,099			33,923
Deferred income taxes	14,338			13,243
Retained earnings	66,608			$ 62,847
Contract revenues	136,513
Costs of contract revenues	120,768
Gross profit	15,745
Income tax expense (benefit)	1,466
Net income (loss)	$ 4,047
Basic (loss) income per share (in dollars per share)	$ 0.14
Diluted (loss) income per share (in dollars per share)	$ 0.14
Accounting Standards Update 2014-09 | ASC 606 Adjustments
Revenue, Major Customer [Line Items]
Costs and estimated earnings in excess of billings on uncompleted contracts	$ (253)		1,383
Billings in excess of costs and estimated earnings on uncompleted contracts	(330)		1,745
Deferred income taxes	23		(76)
Retained earnings	54		$ (286)
Contract revenues	330
Costs of contract revenues	253
Gross profit	77
Income tax expense (benefit)	23
Net income (loss)	$ 54
Basic (loss) income per share (in dollars per share)	$ 0.01
Diluted (loss) income per share (in dollars per share)	$ 0.00