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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
Sep. 30, 2022
Dec. 31, 2021
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 361,158,000 $ 399,680,000
Gross Unrealized Gains (Losses) (42,570,000) 1,491,000
Recorded Basis 318,588,000 401,171,000
Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 83,841,000 96,959,000
Gross Unrealized Gains (Losses) (10,656,000) (196,000)
Recorded Basis 73,185,000 96,763,000
Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 247,530,000 289,337,000
Gross Unrealized Gains (Losses) (31,914,000) 1,687,000
Recorded Basis 215,616,000 291,024,000
Money Market Funds [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 29,787 13,384,000
Recorded Basis 29,787,000 13,384,000
Money Market Funds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 553,000 528,000
Recorded Basis 553,000 528,000
Corporate Bonds [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 38,552,000 41,954,000
Gross Unrealized Gains (Losses) (5,923,000) (136,000)
Recorded Basis 32,629,000 41,818,000
Corporate Bonds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 90,292,000 117,700,000
Gross Unrealized Gains (Losses) (12,900,000) (17,000)
Recorded Basis 77,392,000 117,683,000
U.S. Treasuries [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 66,397,000 67,614,000
Gross Unrealized Gains (Losses) (9,725,000) (342,000)
Recorded Basis 56,672,000 67,272,000
Asset Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 19,496,000 29,533,000
Gross Unrealized Gains (Losses) (1,097,000) (38,000)
Recorded Basis 18,399,000 29,495,000
Asset Backed Securities [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 28,000 107,000
Gross Unrealized Gains (Losses)   1,000
Recorded Basis 28,000 108,000
Mortgage Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 16,366,000 18,089,000
Gross Unrealized Gains (Losses) (3,255,000) (440,000)
Recorded Basis 13,111,000 17,649,000
Mortgage Backed Securities [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 52,309,000 64,217,000
Gross Unrealized Gains (Losses) (6,965,000) 764,000
Recorded Basis 45,344,000 64,981,000
U.S. Government Agency Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 7,373,000 7,383,000
Gross Unrealized Gains (Losses) (301,000) 418,000
Recorded Basis 7,072,000 7,801,000
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 31,827,000 32,898,000
Gross Unrealized Gains (Losses) (2,324,000) 1,281,000
Recorded Basis 29,503,000 34,179,000
Mutual Funds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 6,124,000 6,273,000
Recorded Basis 6,124,000 6,273,000
Emerging Markets [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 2,054,000  
Gross Unrealized Gains (Losses) (80,000)  
Recorded Basis 1,974,000  
Marketable Securities Current [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 29,787,000 13,384,000
Recorded Basis $ 29,787,000 $ 13,384,000