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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Jun. 30, 2024
Dec. 31, 2023
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 290,075 $ 368,753
Gross Unrealized (Losses) Gains (30,878) (28,751)
Recorded Basis 259,197 340,002
Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 3,334 19,539
Recorded Basis 3,334 19,539
Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 77,739 88,496
Gross Unrealized (Losses) Gains (7,639) (7,469)
Recorded Basis 70,100 81,027
Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 209,002 260,718
Gross Unrealized (Losses) Gains (23,239) (21,282)
Recorded Basis 185,763 239,436
Money Market Funds [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 3,334 19,539
Recorded Basis 3,334 19,539
Money Market Funds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 247 337
Recorded Basis 247 337
Corporate Bonds [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 32,258 34,472
Gross Unrealized (Losses) Gains (3,033) (3,159)
Recorded Basis 29,225 31,313
Corporate Bonds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 82,685 82,481
Gross Unrealized (Losses) Gains (8,950) (8,454)
Recorded Basis 73,735 74,027
U.S. Government Agency Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,782 12,830
Gross Unrealized (Losses) Gains (398) (408)
Recorded Basis 12,384 12,422
U.S. Government Agency Securities [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 16,844 16,863
Gross Unrealized (Losses) Gains (1,191) (1,199)
Recorded Basis 15,653 15,664
U.S. Treasuries [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,454 12,448
Gross Unrealized (Losses) Gains (1,348) (1,253)
Recorded Basis 11,106 11,195
U.S. Treasuries [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 54,962 109,020
Gross Unrealized (Losses) Gains (7,016) (6,415)
Recorded Basis 47,946 102,605
Mortgage Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,611 13,084
Gross Unrealized (Losses) Gains (2,727) (2,454)
Recorded Basis 9,884 10,630
Mortgage Backed Securities [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 42,971 42,077
Gross Unrealized (Losses) Gains (6,080) (5,216)
Recorded Basis 36,891 36,861
Asset Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 7,634 13,659
Gross Unrealized (Losses) Gains (133) (187)
Recorded Basis 7,501 13,472
Asset Backed Securities [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 1,094 799
Gross Unrealized (Losses) Gains (2)  
Recorded Basis 1,092 799
Emerging Markets [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost   2,003
Gross Unrealized (Losses) Gains   (8)
Recorded Basis   1,995
Emerging Markets [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 200 200
Gross Unrealized (Losses) Gains   2
Recorded Basis 200 202
Mutual Funds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 9,999 8,941
Recorded Basis $ 9,999 $ 8,941