Construction Contracts - Summary of Backlog (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Remaining Performance Obligation [Roll Forward]
Beginning backlog	$ 30,733	$ 157,722	$ 49,167	$ 217,718
New contracts/change orders	27,807	15,519	32,376	18,960
Work performed	(20,619)	(56,584)	(43,622)	(120,021)
Ending backlog	$ 37,921	$ 116,657	$ 37,921	$ 116,657
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected completion of contracts	12 months		12 months
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected completion of contracts	18 months		18 months