Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investments Current [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	$ 56,175	$ 107,335
Recorded Basis	56,175	107,335
Investments Current [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	1,943	21,312
Recorded Basis	1,943	21,312
Investments Current [Member] | Investments [Member] | Certificates of Deposit [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	4,737
Recorded Basis	4,737
Investments Current [Member] | Investments [Member] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	713
Gross Unrealized Gains (Losses)	1
Recorded Basis	714
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	225
Gross Unrealized Gains (Losses)	(1)
Recorded Basis	224
Investments Current [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	48,557	76,023
Recorded Basis	48,557	76,023
Investments Current [Member] | Restricted Cash and Investments [Member] | Certificates of Deposit [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost		10,000
Recorded Basis		10,000
Investments Noncurrent [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	304,690	301,170
Gross Unrealized Gains (Losses)	(5)	(52)
Recorded Basis	304,685	301,118
Investments Noncurrent [Member] | Investments [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost		4
Gross Unrealized Gains (Losses)		0
Recorded Basis		4
Investments Noncurrent [Member] | Investments [Member] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	76	3,135
Gross Unrealized Gains (Losses)		(3)
Recorded Basis	76	3,132
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	567	2,970
Gross Unrealized Gains (Losses)	(1)	(24)
Recorded Basis	566	2,946
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	248,515	193,835
Gross Unrealized Gains (Losses)	(5)	(52)
Recorded Basis	248,510	193,783
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	236,036	175,869
Recorded Basis	236,036	175,869
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Certificates of Deposit [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	6,047	496
Gross Unrealized Gains (Losses)		(1)
Recorded Basis	6,047	495
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	2,069	3,613
Gross Unrealized Gains (Losses)	(6)	(1)
Recorded Basis	2,063	3,612
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	2,886	2,996
Gross Unrealized Gains (Losses)	2	(24)
Recorded Basis	2,888	2,972
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	834	4,752
Gross Unrealized Gains (Losses)		1
Recorded Basis	$ 834	$ 4,753