Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	$ 367,784	$ 372,953
Gross Unrealized Gains (Losses)	(1,951)	(7,005)
Recorded Basis	365,833	365,948
Investments Current [Member] | Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	45	30
Recorded Basis	45	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	45	30
Recorded Basis	45	30
Investments Current [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	569	417
Gross Unrealized Gains (Losses)		(1)
Recorded Basis	569	416
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	20	20
Recorded Basis	20	20
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	499	347
Recorded Basis	499	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	50	50
Gross Unrealized Gains (Losses)		(1)
Recorded Basis	50	49
Investments Noncurrent [Member] | Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	1,556	1,710
Gross Unrealized Gains (Losses)	(7)	(23)
Recorded Basis	1,549	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	422
Gross Unrealized Gains (Losses)	1	(7)
Recorded Basis	423	415
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	645	794
Gross Unrealized Gains (Losses)		(3)
Recorded Basis	645	791
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	469	484
Gross Unrealized Gains (Losses)	(8)	(13)
Recorded Basis	461	471
Investments Noncurrent [Member] | Investments [Member] | Asset Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	20	10
Recorded Basis	20	10
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	365,614	370,796
Gross Unrealized Gains (Losses)	(1,944)	(6,981)
Recorded Basis	363,670	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	16,371	419
Recorded Basis	16,371	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	178,726	185,116
Gross Unrealized Gains (Losses)	(807)	(3,739)
Recorded Basis	177,919	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	37,583	44,304
Gross Unrealized Gains (Losses)	(168)	(283)
Recorded Basis	37,415	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	43,076	45,548
Gross Unrealized Gains (Losses)	(137)	(908)
Recorded Basis	42,939	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	82,648	89,426
Gross Unrealized Gains (Losses)	(832)	(2,026)
Recorded Basis	81,816	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	130	75
Gross Unrealized Gains (Losses)		(1)
Recorded Basis	130	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	2,264	1,093
Recorded Basis	2,264	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	4,766	4,765
Gross Unrealized Gains (Losses)		(24)
Recorded Basis	4,766	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