Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	$ 334,478	$ 346,087
Gross Unrealized Gains (Losses)	(33,091)	(38,140)
Recorded Basis	301,387	307,947
Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	89,885	77,746
Gross Unrealized Gains (Losses)	(9,797)	(9,421)
Recorded Basis	80,088	68,325
Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	217,770	228,045
Gross Unrealized Gains (Losses)	(23,294)	(28,719)
Recorded Basis	194,476	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	26,823	40,296
Recorded Basis	26,823	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	150	646
Recorded Basis	150	646
Corporate Bonds [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	38,447	38,489
Gross Unrealized Gains (Losses)	(4,644)	(5,181)
Recorded Basis	33,803	33,308
Corporate Bonds [Member] | Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	81,833	81,993
Gross Unrealized Gains (Losses)	(9,834)	(11,296)
Recorded Basis	71,999	70,697
Asset Backed Securities [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	13,696	13,709
Gross Unrealized Gains (Losses)	(581)	(737)
Recorded Basis	13,115	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		9
Recorded Basis		9
Mortgage Backed Securities [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	15,903	16,135
Gross Unrealized Gains (Losses)	(2,972)	(3,139)
Recorded Basis	12,931	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	44,878	46,138
Gross Unrealized Gains (Losses)	(5,394)	(6,045)
Recorded Basis	39,484	40,093
U.S. Treasuries [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	12,439
Gross Unrealized Gains (Losses)	(1,310)
Recorded Basis	11,129
U.S. Treasuries [Member] | Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	56,950	66,479
Gross Unrealized Gains (Losses)	(6,441)	(9,222)
Recorded Basis	50,509	57,257
U.S. Government Agency Securities [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	7,366	7,369
Gross Unrealized Gains (Losses)	(240)	(302)
Recorded Basis	7,126	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	26,450	26,479
Gross Unrealized Gains (Losses)	(1,625)	(2,156)
Recorded Basis	24,825	24,323
Emerging Markets [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	2,034	2,044
Gross Unrealized Gains (Losses)	(50)	(62)
Recorded Basis	1,984	1,982
Mutual Funds [Member] | Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	7,509	6,301
Recorded Basis	7,509	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	26,823	40,296
Recorded Basis	$ 26,823	$ 40,296