Fair Value Measurements (Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Assets:
Restricted Investments	$ 75,382	$ 70,974
Municipal Bonds [Member]
Assets:
Investments in lieu of retainage	1,200	1,200
US Government Agencies Securities [Member]
Assets:
Restricted and other investments	37,500	33,800
Corporate Debt Securities [Member]
Assets:
Restricted and other investments	36,800	35,800
Fair value measured on a recurring basis
Assets:
Cash and cash equivalents	182,599	193,685
Restricted cash	8,892	8,416
Restricted investments	75,382	70,974
Investments in lieu of retainage	100,045	90,791
Total	366,918	363,866
Fair value measured on a recurring basis | Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	182,599	193,685
Restricted cash	8,892	8,416
Investments in lieu of retainage	98,837	89,572
Total	290,328	291,673
Fair value measured on a recurring basis | Significant other observable inputs (Level 2) [Member]
Assets:
Restricted investments	75,382	70,974
Investments in lieu of retainage	1,208	1,219
Total	$ 76,590	$ 72,193
Maximum [Member]
Assets:
Cash and cash equivalents maturity period	3 months
Restricted And other investments, term	5 years	5 years
Certificates of Deposit [Member]
Assets:
Restricted and other investments	$ 1,100	$ 1,400
Money Market Funds [Member]
Assets:
Investments in lieu of retainage	$ 98,800	$ 89,600