Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 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	$ 267,072	$ 380,564
Restricted cash	12,417	14,116
Restricted investments	134,182	130,287
Investments in lieu of retention	124,896	106,949
Total	538,567	631,916
Fair value measured on a recurring basis | Level 1
Assets:
Cash and cash equivalents	267,072	380,564
Restricted cash	12,417	14,116
Restricted investments	0	0
Investments in lieu of retention	31,590	19,988
Total	311,079	414,668
Fair value measured on a recurring basis | Level 1 | Money Market Funds
Assets:
Investments in lieu of retention	31,600	20,000
Fair value measured on a recurring basis | Level 2
Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Restricted investments	134,182	130,287
Investments in lieu of retention	93,306	86,961
Total	227,488	217,248
Fair value measured on a recurring basis | Level 2 | Debt Securities
Assets:
Investments in lieu of retention	93,300	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