Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	$ 418,685	$ 363,671
Gross Unrealized Gains (Losses)	4,783	10,450
Recorded Basis	423,468	374,121
Investments Current [Member] | Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	10,742	42,007
Recorded Basis	10,742	42,007
Investments Current [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	10,742	42,007
Recorded Basis	10,742	42,007
Investments Current [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	102,138	99,729
Gross Unrealized Gains (Losses)	571	1,515
Recorded Basis	102,709	101,244
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	42,007	50,918
Gross Unrealized Gains (Losses)	344	884
Recorded Basis	42,351	51,802
Investments Current [Member] | Investments [Member] | Asset Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	33,548	36,948
Gross Unrealized Gains (Losses)	(25)	(146)
Recorded Basis	33,523	36,802
Investments Current [Member] | Investments [Member] | U.S. Government Agency Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	7,386	7,396
Gross Unrealized Gains (Losses)	548	752
Recorded Basis	7,934	8,148
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		100
Gross Unrealized Gains (Losses)		1
Recorded Basis		101
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	19,197	4,367
Gross Unrealized Gains (Losses)	(296)	24
Recorded Basis	18,901	4,391
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	305,805	221,935
Gross Unrealized Gains (Losses)	4,212	8,935
Recorded Basis	310,017	230,870
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	143	40,063
Gross Unrealized Gains (Losses)		(4)
Recorded Basis	143	40,059
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	127,026	88,902
Gross Unrealized Gains (Losses)	1,209	4,091
Recorded Basis	128,235	92,993
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Asset Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	150	256
Gross Unrealized Gains (Losses)	1	4
Recorded Basis	151	260
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	32,928	29,737
Gross Unrealized Gains (Losses)	1,703	2,466
Recorded Basis	34,631	32,203
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	67,908	4,371
Gross Unrealized Gains (Losses)	(32)	19
Recorded Basis	67,876	4,390
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	71,546	48,795
Gross Unrealized Gains (Losses)	1,331	2,356
Recorded Basis	72,877	51,151
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Mutual Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	6,104	5,036
Recorded Basis	$ 6,104	5,036
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Supranational Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		4,775
Gross Unrealized Gains (Losses)		3
Recorded Basis		$ 4,778