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, 2020	Dec. 31, 2019
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	$ 1,200	$ 1,200
US Government Agencies Securities
Assets:
Restricted and other investments	40,000	33,800
Corporate Debt Securities
Assets:
Restricted and other investments	35,100	35,800
Fair value measured on a recurring basis
Assets:
Cash and cash equivalents	348,366	193,685
Restricted cash	80,974	8,416
Restricted investments	75,475	70,974
Investments in lieu of retainage	107,310	90,791
Total	612,125	363,866
Fair value measured on a recurring basis | Level 1
Assets:
Cash and cash equivalents	348,366	193,685
Restricted cash	80,974	8,416
Restricted investments	0	0
Investments in lieu of retainage	106,068	89,572
Total	535,408	291,673
Fair value measured on a recurring basis | Level 2
Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Restricted investments	75,475	70,974
Investments in lieu of retainage	1,242	1,219
Total	76,717	72,193
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	400	1,400
Money Market Funds
Assets:
Investments in lieu of retainage	$ 106,100	$ 89,600