Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Restricted cash	$ 17,424	$ 50,504
Fair Value Measured On A Recurring Basis [Member]
Assets:
Cash and cash equivalents	221,878	146,103
Restricted cash	17,424	50,504
Investments in lieu of retainage	59,161	51,266
Total assets	298,463	247,873
Fair Value Measured On A Recurring Basis [Member] | Quoted Prices In Active Markets (Level 1) [Member]
Assets:
Cash and cash equivalents	221,878	146,103
Restricted cash	17,424	50,504
Investments in lieu of retainage	56,102	46,855
Total assets	295,404	243,462
Fair Value Measured On A Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Investments in lieu of retainage	3,059	4,411
Total assets	$ 3,059	$ 4,411