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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Jun. 30, 2023
Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 318,184 $ 346,087
Gross Unrealized Gains (Losses) (35,896) (38,140)
Recorded Basis 282,288 307,947
Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 95,240 77,746
Gross Unrealized Gains (Losses) (10,582) (9,421)
Recorded Basis 84,658 68,325
Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 211,280 228,045
Gross Unrealized Gains (Losses) (25,314) (28,719)
Recorded Basis 185,966 199,326
Money Market Funds [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 11,664 40,296
Recorded Basis 11,664 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 28 646
Recorded Basis 28 646
Corporate Bonds [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 38,592 38,489
Gross Unrealized Gains (Losses) (4,891) (5,181)
Recorded Basis 33,701 33,308
Corporate Bonds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 82,628 81,993
Gross Unrealized Gains (Losses) (10,591) (11,296)
Recorded Basis 72,037 70,697
Asset Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 13,684 13,709
Gross Unrealized Gains (Losses) (494) (737)
Recorded Basis 13,190 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 798 9
Gross Unrealized Gains (Losses) (6)  
Recorded Basis 792 9
Mortgage Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 15,622 16,135
Gross Unrealized Gains (Losses) (3,089) (3,139)
Recorded Basis 12,533 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 44,462 46,138
Gross Unrealized Gains (Losses) (5,897) (6,045)
Recorded Basis 38,565 40,093
U.S. Treasuries [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,442  
Gross Unrealized Gains (Losses) (1,522)  
Recorded Basis 10,920  
U.S. Treasuries [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 56,925 66,479
Gross Unrealized Gains (Losses) (7,398) (9,222)
Recorded Basis 49,527 57,257
U.S. Government Agency Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,876 7,369
Gross Unrealized Gains (Losses) (543) (302)
Recorded Basis 12,333 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 18,432 26,479
Gross Unrealized Gains (Losses) (1,422) (2,156)
Recorded Basis 17,010 24,323
Emerging Markets [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 2,024 2,044
Gross Unrealized Gains (Losses) (43) (62)
Recorded Basis 1,981 1,982
Mutual Funds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 8,007 6,301
Recorded Basis 8,007 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 11,664 40,296
Recorded Basis $ 11,664 $ 40,296