Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets:
Restricted cash	$ 52,051	$ 50,504
Restricted investments at amortized cost	9,300
Fair Value Measured On A Recurring Basis [Member]
Assets:
Cash and cash equivalents	172,927	146,103
Restricted cash	52,051	50,504
Investments in lieu of retainage	56,024	51,266
Restricted investments	9,281
Total assets	290,283	247,873
Fair Value Measured On A Recurring Basis [Member] | Quoted Prices In Active Markets (Level 1) [Member]
Assets:
Cash and cash equivalents	172,927	146,103
Restricted cash	52,051	50,504
Investments in lieu of retainage	52,992	46,855
Total assets	277,970	243,462
Fair Value Measured On A Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Investments in lieu of retainage	3,032	4,411
Restricted investments	9,281
Total assets	$ 12,313	$ 4,411