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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2023
Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 343,867 $ 346,087
Gross Unrealized Gains (Losses) (41,019) (38,140)
Recorded Basis 302,848 307,947
Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 99,105 77,746
Gross Unrealized Gains (Losses) (11,452) (9,421)
Recorded Basis 87,653 68,325
Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 234,862 228,045
Gross Unrealized Gains (Losses) (29,567) (28,719)
Recorded Basis 205,295 199,326
U.S. Treasuries [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 8,398  
Recorded Basis 8,398  
U.S. Treasuries [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 16,690  
Gross Unrealized Gains (Losses) (1,683)  
Recorded Basis 15,007  
U.S. Treasuries [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 83,064 66,479
Gross Unrealized Gains (Losses) (8,532) (9,222)
Recorded Basis 74,532 57,257
Money Market Funds [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 1,502 40,296
Recorded Basis 1,502 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 173 646
Recorded Basis 173 646
Corporate Bonds [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 38,525 38,489
Gross Unrealized Gains (Losses) (5,088) (5,181)
Recorded Basis 33,437 33,308
Corporate Bonds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 82,708 81,993
Gross Unrealized Gains (Losses) (11,823) (11,296)
Recorded Basis 70,885 70,697
Asset Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 13,672 13,709
Gross Unrealized Gains (Losses) (341) (737)
Recorded Basis 13,331 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
Gross Unrealized Gains (Losses) (9)  
Recorded Basis 790 9
Mortgage Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 15,351 16,135
Gross Unrealized Gains (Losses) (3,676) (3,139)
Recorded Basis 11,675 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 43,389 46,138
Gross Unrealized Gains (Losses) (7,574) (6,045)
Recorded Basis 35,815 40,093
U.S. Government Agency Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,853 7,369
Gross Unrealized Gains (Losses) (635) (302)
Recorded Basis 12,218 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 16,872 26,479
Gross Unrealized Gains (Losses) (1,629) (2,156)
Recorded Basis 15,243 24,323
Emerging Markets [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 2,014 2,044
Gross Unrealized Gains (Losses) (29) (62)
Recorded Basis 1,985 1,982
Mutual Funds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 7,857 6,301
Recorded Basis 7,857 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 9,900 40,296
Recorded Basis $ 9,900 $ 40,296