Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Restricted cash	$ 4,090	$ 4,780
Fair Value Measured On A Recurring Basis [Member]
Assets:
Cash and cash equivalents	174,340	192,868
Restricted cash	4,090	4,780
Investments in lieu of retainage	67,809	72,296
Total assets	246,239	269,944
Fair Value Measured On A Recurring Basis [Member] | Quoted Prices In Active Markets (Level 1) [Member]
Assets:
Cash and cash equivalents	174,340	192,868
Restricted cash	4,090	4,780
Investments in lieu of retainage	65,708	69,891
Total assets	244,138	267,539
Fair Value Measured On A Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Investments in lieu of retainage	2,101	2,405
Total assets	$ 2,101	$ 2,405