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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2024
Dec. 31, 2023
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 326,720 $ 368,753
Gross Unrealized (Losses) Gains (21,474) (28,751)
Recorded Basis 305,246 340,002
Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 4,873 19,539
Recorded Basis 4,873 19,539
Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 85,331 88,496
Gross Unrealized (Losses) Gains (5,397) (7,469)
Recorded Basis 79,934 81,027
Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 236,516 260,718
Gross Unrealized (Losses) Gains (16,077) (21,282)
Recorded Basis 220,439 239,436
Money Market Funds [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 4,873 19,539
Recorded Basis 4,873 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 99 337
Recorded Basis 99 337
Corporate Bonds [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 32,223 34,472
Gross Unrealized (Losses) Gains (2,092) (3,159)
Recorded Basis 30,131 31,313
Corporate Bonds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 100,340 82,481
Gross Unrealized (Losses) Gains (6,037) (8,454)
Recorded Basis 94,303 74,027
U.S. Treasuries [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 20,380 12,448
Gross Unrealized (Losses) Gains (924) (1,253)
Recorded Basis 19,456 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 65,028 109,020
Gross Unrealized (Losses) Gains (5,045) (6,415)
Recorded Basis 59,983 102,605
U.S. Government Agency Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,758 12,830
Gross Unrealized (Losses) Gains (176) (408)
Recorded Basis 12,582 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 16,795 16,863
Gross Unrealized (Losses) Gains (706) (1,199)
Recorded Basis 16,089 15,664
Mortgage Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,348 13,084
Gross Unrealized (Losses) Gains (2,115) (2,454)
Recorded Basis 10,233 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 41,942 42,077
Gross Unrealized (Losses) Gains (4,309) (5,216)
Recorded Basis 37,633 36,861
Asset Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 7,622 13,659
Gross Unrealized (Losses) Gains (90) (187)
Recorded Basis 7,532 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 1,367 799
Gross Unrealized (Losses) Gains 11  
Recorded Basis 1,378 799
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 9 2
Recorded Basis 209 202
Mutual Funds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 10,745 8,941
Recorded Basis $ 10,745 $ 8,941