Summary of Significant Accounting Principles - Prior Period Adjustment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income (loss)	$ 4,101	$ (1,808)
Contract revenues	136,843	138,757
Costs of contract revenues	121,021	125,772
Selling, general and administrative expenses	15,014	14,979
Income tax expense (benefit)	$ 1,489	$ (1,019)