Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Assets:
Cash and cash equivalents maturity period (maximum)	3 months
Fair value measured on a recurring basis
Assets:
Cash and cash equivalents	$ 358,304	$ 380,564
Restricted cash	14,749	14,116
Restricted investments	130,499	130,287
Investments in lieu of retention	115,956	106,949
Total	619,508	631,916
Fair value measured on a recurring basis | Level 1
Assets:
Cash and cash equivalents	358,304	380,564
Restricted cash	14,749	14,116
Restricted investments	0	0
Investments in lieu of retention	28,342	19,988
Total	401,395	414,668
Fair value measured on a recurring basis | Level 1 | Money Market Funds
Assets:
Investments in lieu of retention	28,300	20,000
Fair value measured on a recurring basis | Level 2
Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Restricted investments	130,499	130,287
Investments in lieu of retention	87,614	86,961
Total	218,113	217,248
Fair value measured on a recurring basis | Level 2 | Debt Securities
Assets:
Investments in lieu of retention	87,600	87,000
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 retention	0	0
Total	$ 0	$ 0