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, 2021	Dec. 31, 2020
Assets:
Cash and cash equivalents maturity period (maximum)	3 months
Restricted and other investments, term (maximum)	5 years	5 years
Municipal Bonds
Assets:
Investments in lieu of retainage	$ 1,300	$ 1,300
US Government Agencies Securities
Assets:
Restricted and other investments	40,700	40,500
Corporate Debt Securities
Assets:
Investments in lieu of retainage	53,900
Restricted and other investments	43,900	37,500
Fair value measured on a recurring basis
Assets:
Cash and cash equivalents	231,129	374,289
Restricted cash	2,884	77,563
Restricted investments	85,545	78,912
Investments in lieu of retainage	100,977	93,909
Total	420,535	624,673
Fair value measured on a recurring basis | Level 1
Assets:
Cash and cash equivalents	231,129	374,289
Restricted cash	2,884	77,563
Restricted investments	0	0
Investments in lieu of retainage	45,803	92,609
Total	279,816	544,461
Fair value measured on a recurring basis | Level 2
Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Restricted investments	85,545	78,912
Investments in lieu of retainage	55,174	1,300
Total	140,719	80,212
Fair value measured on a recurring basis | Level 3
Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Restricted investments	0	0
Investments in lieu of retainage	0	0
Total	0	0
Certificates of Deposit
Assets:
Restricted and other investments	900	900
Money Market Funds
Assets:
Investments in lieu of retainage	$ 45,800	$ 92,600