Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Municipal Bonds
Assets:
Investments in lieu of retainage	$ 1,300	$ 1,300
Fair Value, Measurements, Recurring
Assets:
Cash and cash equivalents	202,197	374,289
Restricted cash	9,199	77,563
Restricted investments	84,355	78,912
Investments in lieu of retainage	86,328	93,909
Total	382,079	624,673
Fair Value, Measurements, Recurring | Level 1
Assets:
Cash and cash equivalents	202,197	374,289
Restricted cash	9,199	77,563
Restricted investments	0	0
Investments in lieu of retainage	27,472	92,609
Total	238,868	544,461
Fair Value, Measurements, Recurring | Level 2
Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Restricted investments	84,355	78,912
Investments in lieu of retainage	58,856	1,300
Total	143,211	80,212
Fair Value, Measurements, Recurring | 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
Maximum
Assets:
Restricted investment maturity period	5 years	5 years
Maximum | Corporate and Municipal Bonds
Assets:
Restricted investment maturity period	5 years
US Government Agencies Securities
Assets:
Restricted and other investments	$ 37,100	$ 40,500
Corporate Debt Securities
Assets:
Investments in lieu of retainage	57,500
Restricted and other investments	46,700	37,500
Certificates of Deposit
Assets:
Restricted and other investments	600	900
Money Market Funds
Assets:
Investments in lieu of retainage	$ 27,500	$ 92,600