Contract Assets and Liabilities - Components of Contract Asset and Liability Balances (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Contract assets
Costs in excess of billings and estimated earnings	$ 38,200		$ 31,894
Retainage receivable	32,615		35,204
Total contract assets	70,815		67,098
Change in costs in excess of billings and estimated earnings	6,306
Change in retainage receivable	(2,589)
Change in contract assets	3,717	$ (4,901)
Contract liabilities
Billings in excess of costs and estimated earnings	38,611		46,020
Provisions for losses	568		628
Total contract liabilities	39,179		$ 46,648
Change in billings in excess of costs and estimated earnings	(7,409)
Change in provisions for losses	(60)
Change in contract liabilities	$ (7,469)	$ 16,254