Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	$ 349,980	$ 372,953
Gross Unrealized Gains (Losses)	2,256	(7,005)
Recorded Basis	352,236	365,948
Investments Current [Member] | Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	224	30
Recorded Basis	224	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	224	30
Recorded Basis	224	30
Investments Current [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	77,203	417
Gross Unrealized Gains (Losses)	68	(1)
Recorded Basis	77,271	416
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	48,822	20
Gross Unrealized Gains (Losses)	(126)
Recorded Basis	48,696	20
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	19,002	347
Gross Unrealized Gains (Losses)	(50)
Recorded Basis	18,952	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	8,913	50
Gross Unrealized Gains (Losses)	248	(1)
Recorded Basis	9,161	49
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	447
Gross Unrealized Gains (Losses)	(4)
Recorded Basis	443
Investments Current [Member] | Investments [Member] | Asset Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	19
Recorded Basis	19
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] | 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] | 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] | Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	272,553	370,796
Gross Unrealized Gains (Losses)	2,188	(6,981)
Recorded Basis	274,741	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	16,614	419
Recorded Basis	16,614	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	128,012	185,116
Gross Unrealized Gains (Losses)	1,265	(3,739)
Recorded Basis	129,277	181,377
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	18,794	44,304
Gross Unrealized Gains (Losses)	21	(283)
Recorded Basis	18,815	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	29,801	45,548
Gross Unrealized Gains (Losses)	675	(908)
Recorded Basis	30,476	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	71,839	89,426
Gross Unrealized Gains (Losses)	192	(2,026)
Recorded Basis	72,031	87,400
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	126	75
Gross Unrealized Gains (Losses)	1	(1)
Recorded Basis	127	74
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	2,550	1,093
Recorded Basis	2,550	1,093
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	4,767	4,765
Gross Unrealized Gains (Losses)	34	(24)
Recorded Basis	4,801	4,741
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	50
Recorded Basis	$ 50	$ 50