Fair Value Measurements (Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 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	40,000	40,500
Corporate Debt Securities
Assets:
Investments in lieu of retainage	55,300
Restricted and other investments	45,800	37,500
Fair value measured on a recurring basis
Assets:
Cash and cash equivalents	187,535	374,289
Restricted cash	6,922	77,563
Restricted investments	86,576	78,912
Investments in lieu of retainage	82,193	93,909
Total	363,226	624,673
Fair value measured on a recurring basis | Level 1
Assets:
Cash and cash equivalents	187,535	374,289
Restricted cash	6,922	77,563
Restricted investments	0	0
Investments in lieu of retainage	25,638	92,609
Total	220,095	544,461
Fair value measured on a recurring basis | Level 2
Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Restricted investments	86,576	78,912
Investments in lieu of retainage	56,555	1,300
Total	143,131	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	800	900
Money Market Funds
Assets:
Investments in lieu of retainage	$ 25,600	$ 92,600