Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	$ 489,744	$ 363,671
Gross Unrealized Gains (Losses)	6,172	10,450
Recorded Basis	495,916	374,121
Investments Current [Member] | Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	1,149	42,007
Recorded Basis	1,149	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	1,149	42,007
Recorded Basis	1,149	42,007
Investments Current [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	106,268	99,729
Gross Unrealized Gains (Losses)	1,016	1,515
Recorded Basis	107,284	101,244
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	48,243	50,918
Gross Unrealized Gains (Losses)	541	884
Recorded Basis	48,784	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	36,943	36,948
Gross Unrealized Gains (Losses)	(7)	(146)
Recorded Basis	36,936	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	7,389	7,396
Gross Unrealized Gains (Losses)	593	752
Recorded Basis	7,982	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	2,101	100
Gross Unrealized Gains (Losses)	14	1
Recorded Basis	2,115	101
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	11,592	4,367
Gross Unrealized Gains (Losses)	(125)	24
Recorded Basis	11,467	4,391
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	382,327	221,935
Gross Unrealized Gains (Losses)	5,156	8,935
Recorded Basis	387,483	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	396	40,063
Gross Unrealized Gains (Losses)		(4)
Recorded Basis	396	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	183,186	88,902
Gross Unrealized Gains (Losses)	1,628	4,091
Recorded Basis	184,814	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	203	256
Gross Unrealized Gains (Losses)	2	4
Recorded Basis	205	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	34,908	29,737
Gross Unrealized Gains (Losses)	1,909	2,466
Recorded Basis	36,817	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	69,225	4,371
Gross Unrealized Gains (Losses)	147	19
Recorded Basis	69,372	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	88,288	48,795
Gross Unrealized Gains (Losses)	1,470	2,356
Recorded Basis	89,758	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	6,121	5,036
Recorded Basis	$ 6,121	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