Fair Value Measurement - Summary of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	$ 343,937	$ 304,690
Gross Unrealized Gains (Losses)	185	(5)
Recorded Basis	344,122	304,685
Investments Current [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	1,488	7,618
Gross Unrealized Gains (Losses)	1
Recorded Basis	1,489	7,618
Investments Current [Member] | Cash and Cash Equivalents [Member] | U.S. Government Agency Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	500
Recorded Basis	500
Investments Current [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	24	1,943
Recorded Basis	24	1,943
Investments Current [Member] | Investments [Member] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	320	713
Gross Unrealized Gains (Losses)	1	1
Recorded Basis	321	714
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	644	225
Gross Unrealized Gains (Losses)		(1)
Recorded Basis	644	224
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 Noncurrent [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	591	643
Gross Unrealized Gains (Losses)	(1)	(1)
Recorded Basis	590	642
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	202
Recorded Basis	202
Investments Noncurrent [Member] | Investments [Member] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	61	76
Recorded Basis	61	76
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	328	567
Gross Unrealized Gains (Losses)	(1)	(1)
Recorded Basis	327	566
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	341,858	296,429
Gross Unrealized Gains (Losses)	185	(4)
Recorded Basis	342,043	296,425
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	26,697	284,593
Recorded Basis	26,697	284,593
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	38,510	834
Gross Unrealized Gains (Losses)	(23)
Recorded Basis	38,487	834
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	5,558	2,069
Gross Unrealized Gains (Losses)	28	(6)
Recorded Basis	5,586	2,063
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	144,321	2,886
Gross Unrealized Gains (Losses)	138	2
Recorded Basis	144,459	2,888
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Certificates of Deposit [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost		6,047
Recorded Basis		$ 6,047
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	54,554
Recorded Basis	54,554
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	72,218
Gross Unrealized Gains (Losses)	42
Recorded Basis	$ 72,260