Summary of Significant Accounting Policies (Costs and Estimated Earnings in Excess of Billings) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Summary of Significant Accounting Policies [Abstract]
Claims	$ 549,849	$ 477,425
Unapproved change orders	296,591	207,475
Other unbilled costs and profits	86,318	146,926
Total costs and estimated earnings in excess of billings	$ 932,758	$ 831,826