Contract Assets and Liabilities - Components of Contract Asset and Liability Balances (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Contract assets
Costs in excess of billings and estimated earnings	$ 39,898		$ 44,315
Retainage receivable	32,389		32,873
Total contract assets	72,287	$ 68,285	77,188
Change in costs in excess of billings and estimated earnings	(4,417)
Change in retainage receivable	(484)
Change in total contract assets	(4,901)	4,476
Contract liabilities
Billings in excess of costs and estimated earnings	57,874		40,662
Provisions for losses	750		1,708
Total contract liabilities	58,624		$ 42,370
Change in billings in excess of costs and estimated earnings	17,212
Change in provisions for losses	(958)
Change in total contract liabilities	$ 16,254	$ (4,686)