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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
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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

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