Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Municipal Bonds
Assets:
Investments in lieu of retainage	$ 1,300	$ 1,200
Fair Value, Measurements, Recurring
Assets:
Cash and cash equivalents	374,289	193,685
Restricted cash	77,563	8,416
Restricted investments	78,912	70,974
Investments in lieu of retainage	93,909	90,791
Total	624,673	363,866
Fair Value, Measurements, Recurring | Level 1
Assets:
Cash and cash equivalents	374,289	193,685
Restricted cash	77,563	8,416
Restricted investments	0	0
Investments in lieu of retainage	92,609	89,572
Total	544,461	291,673
Fair Value, Measurements, Recurring | Level 2
Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Restricted investments	78,912	70,974
Investments in lieu of retainage	1,300	1,219
Total	80,212	72,193
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
Corporate Debt Securities
Assets:
Restricted and other investments	$ 37,500	$ 35,800
US Government Agencies Securities
Assets:
Restricted and other investments	40,500	33,800
Certificates of Deposit
Assets:
Restricted and other investments	900	1,400
Money Market Funds
Assets:
Investments in lieu of retainage	$ 92,600	$ 89,600