Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis)(Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Restricted cash	$ 4,780	$ 50,504
Fair Value, Measurements, Recurring [Member]
Assets:
Cash and cash equivalents	192,868	146,103
Restricted cash	4,780	50,504
Investments in lieu of retainage	72,296	51,266
Total	269,944	247,873
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	192,868	146,103
Restricted cash	4,780	50,504
Investments in lieu of retainage	69,891	46,855
Total	267,539	243,462
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Investments in lieu of retainage	2,405	4,411
Total	2,405	$ 4,411
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Cash and cash equivalents
Restricted cash
Investments in lieu of retainage
Total