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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Dec. 31, 2023
Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 368,753 $ 346,087
Unrealized Gains (Losses) (28,751) (38,140)
Recorded Basis 340,002 307,947
Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 88,496 77,746
Unrealized Gains (Losses) (7,469) (9,421)
Recorded Basis 81,027 68,325
Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 260,718 228,045
Unrealized Gains (Losses) (21,282) (28,719)
Recorded Basis 239,436 199,326
Money Market Funds [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 19,539 40,296
Recorded Basis 19,539 40,296
Money Market Funds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 337 646
Recorded Basis 337 646
Corporate Bonds [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 34,472 38,489
Unrealized Gains (Losses) (3,159) (5,181)
Recorded Basis 31,313 33,308
Corporate Bonds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 82,481 81,993
Unrealized Gains (Losses) (8,454) (11,296)
Recorded Basis 74,027 70,697
Asset Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 13,659 13,709
Unrealized Gains (Losses) (187) (737)
Recorded Basis 13,472 12,972
Asset Backed Securities [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 799 9
Recorded Basis 799 9
U.S. Government Agency Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,830 7,369
Unrealized Gains (Losses) (408) (302)
Recorded Basis 12,422 7,067
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 16,863 26,479
Unrealized Gains (Losses) (1,199) (2,156)
Recorded Basis 15,664 24,323
U.S. Treasuries [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,448  
Unrealized Gains (Losses) (1,253)  
Recorded Basis 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 109,020 66,479
Unrealized Gains (Losses) (6,415) (9,222)
Recorded Basis 102,605 57,257
Mortgage Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 13,084 16,135
Unrealized Gains (Losses) (2,454) (3,139)
Recorded Basis 10,630 12,996
Mortgage Backed Securities [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 42,077 46,138
Unrealized Gains (Losses) (5,216) (6,045)
Recorded Basis 36,861 40,093
Emerging Markets [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 2,003 2,044
Unrealized Gains (Losses) (8) (62)
Recorded Basis 1,995 1,982
Emerging Markets [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 200  
Unrealized Gains (Losses) 2  
Recorded Basis 202  
Mutual Funds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 8,941 6,301
Recorded Basis 8,941 6,301
Marketable Securities Current [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 19,539 $ 40,296