Costs and Estimated Earnings on Uncompleted Contracts (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Costs and Estimated Earnings on Uncompleted Contracts
Costs incurred on uncompleted contracts	$ 5,475,483	$ 5,391,124
Gross profit recognized	458,018	456,871
Costs and Estimated Earnings on Uncompleted Contracts	5,933,501	5,847,995
Less: billings to date	(5,914,959)	(5,821,983)
Net cost and estimated earnings in excess of billings	18,542	26,012
Amount included in consolidated balance sheet
Costs and estimated earnings in excess of billings	177,662	138,618
Billings in excess of costs and estimated earnings	(159,120)	(112,606)
Net cost and estimated earnings in excess of billings	$ 18,542	$ 26,012