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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	$ 363,671	$ 361,468
Gross Unrealized Gains (Losses)	10,450	3,896
Recorded Basis	374,121	365,364
Investments Current [Member] | Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	42,007	35,526
Recorded Basis	42,007	35,526
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	42,007	35,526
Recorded Basis	42,007	35,526
Investments Current [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	99,729	82,155
Gross Unrealized Gains (Losses)	1,515	435
Recorded Basis	101,244	82,590
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	50,918	53,493
Gross Unrealized Gains (Losses)	884	156
Recorded Basis	51,802	53,649
Investments Current [Member] | Investments [Member] | Asset Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	36,948	14,017
Gross Unrealized Gains (Losses)	(146)	(13)
Recorded Basis	36,802	14,004
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	7,396	7,408
Gross Unrealized Gains (Losses)	752	282
Recorded Basis	8,148	7,690
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	4,367	2,737
Gross Unrealized Gains (Losses)	24	1
Recorded Basis	4,391	2,738
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	100	4,500
Gross Unrealized Gains (Losses)	1	9
Recorded Basis	101	4,509
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	221,935	243,787
Gross Unrealized Gains (Losses)	8,935	3,461
Recorded Basis	230,870	247,248
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	40,063	29,046
Gross Unrealized Gains (Losses)	(4)
Recorded Basis	40,059	29,046
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	88,902	98,481
Gross Unrealized Gains (Losses)	4,091	1,931
Recorded Basis	92,993	100,412
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	256	303
Gross Unrealized Gains (Losses)	4
Recorded Basis	260	303
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	29,737	27,885
Gross Unrealized Gains (Losses)	2,466	642
Recorded Basis	32,203	28,527
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	48,795	62,930
Gross Unrealized Gains (Losses)	2,356	837
Recorded Basis	51,151	63,767
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	4,371	16,906
Gross Unrealized Gains (Losses)	19	21
Recorded Basis	4,390	16,927
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	5,036	3,466
Recorded Basis	5,036	3,466
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	4,770
Gross Unrealized Gains (Losses)	3	30
Recorded Basis	$ 4,778	$ 4,800

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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