Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis)(Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets:
Restricted cash	$ 50,504	$ 45,853
Recurring [Member]
Assets:
Cash and cash equivalents	146,103	75,452
Restricted cash	50,504	45,853
Investments in lieu of customer retainage	51,266	41,566
Total	247,873	162,871
Liabilities:
Interest rate swap contract		45
Total		45
Recurring [Member] | Level 1 [Member]
Assets:
Cash and cash equivalents	146,103	75,452
Restricted cash	50,504	45,853
Investments in lieu of customer retainage	46,855	35,350
Total	243,462	156,655
Recurring [Member] | Level 2 [Member]
Assets:
Investments in lieu of customer retainage	4,411	6,216
Total	4,411	6,216
Liabilities:
Interest rate swap contract		45
Total		$ 45
Recurring [Member] | Level 3 [Member]
Assets:
Cash and cash equivalents
Restricted cash
Investments in lieu of customer retainage
Total
Liabilities:
Interest rate swap contract
Total