Costs and estimated earnings in excess of billings (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Costs and estimated earnings in excess of billings [Abstract]
Unbilled costs and profits incurred to date	[1]	$ 275,079	$ 253,078
Unapproved change orders		312,090	161,375
Claims		296,640	311,949
Costs and estimated earnings in excess of billings		883,809	726,402
Unapproved change orders and claims subject to pending litigation		$ 41,500	$ 38,400

[1] Represents the excess of contract costs and profits recognized to date on the percentage of completion accounting method over the amount of contract billings to date on certain contracts.