Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Municipal Bonds [Member]
Assets:
Investments in lieu of retainage	$ 1,200	$ 1,200
Money Market Funds [Member]
Assets:
Investments in lieu of retainage	71,800	62,900
US Government Agencies Debt Securities [Member]
Assets:
Restricted and other investments	28,300	27,700
Corporate Debt Securities [Member]
Assets:
Restricted and other investments	37,000	30,400
Fair Value Measured On A Recurring Basis [Member]
Assets:
Cash and cash equivalents	149,881	116,075
Restricted cash	4,742	3,788
Restricted investments	65,287	58,142
Investments in lieu of retainage	73,014	64,048
Total assets	292,924	242,053
Fair Value Measured On A Recurring Basis [Member] | Quoted Prices In Active Markets (Level 1) [Member]
Assets:
Cash and cash equivalents	149,881	116,075
Restricted cash	4,742	3,788
Investments in lieu of retainage	71,828	62,858
Total assets	226,451	182,721
Fair Value Measured On A Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Restricted investments	65,287	58,142
Investments in lieu of retainage	1,186	1,190
Total assets	$ 66,473	$ 59,332
Maximum [Member]
Assets:
Restricted And other investments, term	5 years