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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	$ 372,953	$ 392,615
Gross Unrealized Gains (Losses)	(7,005)	(1,936)
Recorded Basis	365,948	390,679
Investments Current [Member] | Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	30	221
Recorded Basis	30	221
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	30	121
Recorded Basis	30	121
Investments Current [Member] | Cash and Cash Equivalents [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		100
Recorded Basis		100
Investments Current [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	417	674
Gross Unrealized Gains (Losses)	(1)
Recorded Basis	416	674
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	347	199
Recorded Basis	347	199
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	20	400
Recorded Basis	20	400
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	50	65
Gross Unrealized Gains (Losses)	(1)
Recorded Basis	49	65
Investments Current [Member] | Investments [Member] | Municipal Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		10
Recorded Basis		10
Investments Noncurrent [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	1,710	1,208
Gross Unrealized Gains (Losses)	(23)	(9)
Recorded Basis	1,687	1,199
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	794	202
Gross Unrealized Gains (Losses)	(3)	(2)
Recorded Basis	791	200
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	422	419
Gross Unrealized Gains (Losses)	(7)	(2)
Recorded Basis	415	417
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	484	577
Gross Unrealized Gains (Losses)	(13)	(5)
Recorded Basis	471	572
Investments Noncurrent [Member] | Investments [Member] | Asset Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	10	10
Recorded Basis	10	10
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	370,796	390,512
Gross Unrealized Gains (Losses)	(6,981)	(1,927)
Recorded Basis	363,815	388,585
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	419	16,018
Recorded Basis	419	16,018
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	44,304	45,833
Gross Unrealized Gains (Losses)	(283)	(143)
Recorded Basis	44,021	45,690
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	185,116	184,808
Gross Unrealized Gains (Losses)	(3,739)	(953)
Recorded Basis	181,377	183,855
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	45,548	38,168
Gross Unrealized Gains (Losses)	(908)	(222)
Recorded Basis	44,640	37,946
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	472
Gross Unrealized Gains (Losses)		(14)
Recorded Basis	50	458
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	89,426	86,240
Gross Unrealized Gains (Losses)	(2,026)	(595)
Recorded Basis	87,400	85,645
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	75
Gross Unrealized Gains (Losses)	(1)
Recorded Basis	74
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Supranational [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	4,765
Gross Unrealized Gains (Losses)	(24)
Recorded Basis	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	1,093
Recorded Basis	$ 1,093
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		18,973
Recorded Basis		$ 18,973

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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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