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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	$ 361,468	$ 372,953
Gross Unrealized Gains (Losses)	3,896	(7,005)
Recorded Basis	365,364	365,948
Investments Current [Member] | Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	35,526	30
Recorded Basis	35,526	30
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	35,526	30
Recorded Basis	35,526	30
Investments Current [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	82,155	417
Gross Unrealized Gains (Losses)	435	(1)
Recorded Basis	82,590	416
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	53,493	20
Gross Unrealized Gains (Losses)	156
Recorded Basis	53,649	20
Investments Current [Member] | Investments [Member] | Asset Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	14,017
Gross Unrealized Gains (Losses)	(13)
Recorded Basis	14,004
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	4,500	347
Gross Unrealized Gains (Losses)	9
Recorded Basis	4,509	347
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	7,408	50
Gross Unrealized Gains (Losses)	282	(1)
Recorded Basis	7,690	49
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	2,737
Gross Unrealized Gains (Losses)	1
Recorded Basis	2,738
Investments Noncurrent [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		1,710
Gross Unrealized Gains (Losses)		(23)
Recorded Basis		1,687
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		422
Gross Unrealized Gains (Losses)		(7)
Recorded Basis		415
Investments Noncurrent [Member] | Investments [Member] | Asset Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		10
Recorded Basis		10
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		794
Gross Unrealized Gains (Losses)		(3)
Recorded Basis		791
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		484
Gross Unrealized Gains (Losses)		(13)
Recorded Basis		471
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	243,787	370,796
Gross Unrealized Gains (Losses)	3,461	(6,981)
Recorded Basis	247,248	363,815
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	29,046	419
Recorded Basis	29,046	419
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	98,481	185,116
Gross Unrealized Gains (Losses)	1,931	(3,739)
Recorded Basis	100,412	181,377
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	303	75
Gross Unrealized Gains (Losses)		(1)
Recorded Basis	303	74
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	16,906	44,304
Gross Unrealized Gains (Losses)	21	(283)
Recorded Basis	16,927	44,021
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	27,885	45,548
Gross Unrealized Gains (Losses)	642	(908)
Recorded Basis	28,527	44,640
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	62,930	89,426
Gross Unrealized Gains (Losses)	837	(2,026)
Recorded Basis	63,767	87,400
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	4,770	4,765
Gross Unrealized Gains (Losses)	30	(24)
Recorded Basis	4,800	4,741
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	3,466	1,093
Recorded Basis	$ 3,466	1,093
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Municipal Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		50
Recorded Basis		$ 50

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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).

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