Summary of Significant Accounting Principles - Prior Period Adjustment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income (loss)	$ 2,249	$ (2,293)	$ 6,350	$ (4,101)
Contract revenues	159,767	137,420	296,610	276,177
Costs of contract revenues	138,998	122,023	260,019	247,795
Selling, general and administrative expenses	16,864	17,528	31,878	32,507
Income tax expense (benefit)	$ 1,660	$ (1,624)	$ 3,149	$ (2,643)