Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Assets:
Restricted cash	$ 3,434	$ 4,780
Restricted and other investments, amortized cost	60,100
Amortized cost of investments held to maturity		59,600
Fair value of investments held to maturity		59,100
Municipal Bonds [Member]
Assets:
Investments in lieu of retainage	2,100	2,400
Money Market Funds [Member]
Assets:
Investments in lieu of retainage	63,400	69,900
US Government Agencies Debt Securities [Member]
Assets:
Restricted and other investments	27,100	26,100
Corporate Debt Securities [Member]
Assets:
Restricted and other investments	31,700	33,000
Fair Value Measured On A Recurring Basis [Member]
Assets:
Cash and cash equivalents	138,569	192,868
Restricted cash	3,434	4,780
Restricted investments	52,900
Investments in lieu of retainage	65,535	72,296
Other investments	5,949
Total assets	266,387	269,944
Fair Value Measured On A Recurring Basis [Member] | Quoted Prices In Active Markets (Level 1) [Member]
Assets:
Cash and cash equivalents	138,569	192,868
Restricted cash	3,434	4,780
Investments in lieu of retainage	63,441	69,891
Total assets	205,444	267,539
Fair Value Measured On A Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Restricted investments	52,900
Investments in lieu of retainage	2,094	2,405
Other investments	5,949
Total assets	$ 60,943	$ 2,405
Minimum [Member[
Assets:
Restricted And other investments, term	1 month
Maximum [Member]
Assets:
Restricted And other investments, term	5 years