Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	$ 413,450	$ 392,615
Gross Unrealized Gains (Losses)	(8,803)	(1,936)
Recorded Basis	404,647	390,679
Investments Current [Member] | Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	56	221
Recorded Basis	56	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	56	121
Recorded Basis	56	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	465	674
Gross Unrealized Gains (Losses)	(1)
Recorded Basis	464	674
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	344	199
Recorded Basis	344	199
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	61	400
Gross Unrealized Gains (Losses)	(1)
Recorded Basis	60	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	60	65
Recorded Basis	60	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	1,610	1,208
Gross Unrealized Gains (Losses)	(32)	(9)
Recorded Basis	1,578	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	645	202
Gross Unrealized Gains (Losses)	(7)	(2)
Recorded Basis	638	200
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	384	419
Gross Unrealized Gains (Losses)	(11)	(2)
Recorded Basis	373	417
Investments Noncurrent [Member] | Investments [Member] | U.S. Government Agency Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	49
Recorded Basis	49
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost	522	577
Gross Unrealized Gains (Losses)	(14)	(5)
Recorded Basis	508	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	411,319	390,512
Gross Unrealized Gains (Losses)	(8,770)	(1,927)
Recorded Basis	402,549	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	361	16,018
Recorded Basis	361	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	72,776	45,833
Gross Unrealized Gains (Losses)	(365)	(143)
Recorded Basis	72,411	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	190,230	184,808
Gross Unrealized Gains (Losses)	(4,512)	(953)
Recorded Basis	185,718	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	45,592	38,168
Gross Unrealized Gains (Losses)	(1,233)	(222)
Recorded Basis	44,359	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	185	472
Gross Unrealized Gains (Losses)		(14)
Recorded Basis	185	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	97,337	86,240
Gross Unrealized Gains (Losses)	(2,610)	(595)
Recorded Basis	94,727	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] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cost		18,973
Recorded Basis		$ 18,973
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	4,763
Gross Unrealized Gains (Losses)	(49)
Recorded Basis	$ 4,714