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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Mar. 31, 2024
Dec. 31, 2023
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 376,567 $ 368,753
Gross Unrealized (Losses) Gains (30,818) (28,751)
Recorded Basis 345,749 340,002
Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 18,549 19,539
Recorded Basis 18,549 19,539
Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 80,131 88,496
Gross Unrealized (Losses) Gains (7,721) (7,469)
Recorded Basis 72,410 81,027
Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 277,887 260,718
Gross Unrealized (Losses) Gains (23,097) (21,282)
Recorded Basis 254,790 239,436
Money Market Funds [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 18,549 19,539
Recorded Basis 18,549 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 107 337
Recorded Basis 107 337
Corporate Bonds [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 32,319 34,472
Gross Unrealized (Losses) Gains (3,123) (3,159)
Recorded Basis 29,196 31,313
Corporate Bonds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 82,816 82,481
Gross Unrealized (Losses) Gains (9,022) (8,454)
Recorded Basis 73,794 74,027
U.S. Government Agency Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,806 12,830
Gross Unrealized (Losses) Gains (395) (408)
Recorded Basis 12,411 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 16,854 16,863
Gross Unrealized (Losses) Gains (1,170) (1,199)
Recorded Basis 15,684 15,664
U.S. Treasuries [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,451 12,448
Gross Unrealized (Losses) Gains (1,369) (1,253)
Recorded Basis 11,082 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 125,052 109,020
Gross Unrealized (Losses) Gains (7,032) (6,415)
Recorded Basis 118,020 102,605
Mortgage Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 12,908 13,084
Gross Unrealized (Losses) Gains (2,677) (2,454)
Recorded Basis 10,231 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 42,418 42,077
Gross Unrealized (Losses) Gains (5,874) (5,216)
Recorded Basis 36,544 36,861
Asset Backed Securities [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 9,647 13,659
Gross Unrealized (Losses) Gains (157) (187)
Recorded Basis 9,490 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 799 799
Gross Unrealized (Losses) Gains (2)  
Recorded Basis 797 799
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 3 2
Recorded Basis 203 202
Mutual Funds [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 9,641 8,941
Recorded Basis $ 9,641 $ 8,941