Contract Assets and Liabilities - Components of Contract Asset and Liability Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Contract assets
Costs in excess of billings and estimated earnings	$ 31,894	$ 44,315
Retainage receivable	35,204	32,873
Total contract assets	67,098	77,188
Change in costs in excess of billings and estimated earnings	(12,421)
Change in retainage receivable	2,331
Change in total contract assets	(10,090)	13,378
Contract liabilities
Billings in excess of costs and estimated earnings on uncompleted contracts	46,020	40,662
Provisions for losses	628	1,708
Total contract liabilities	46,648	42,370
Change in billings in excess of costs and estimated earnings	5,358
Change in provisions for losses	(1,080)
Change in total contract liabilities	$ 4,278	$ (6,446)