Fair Value Measurement - Summary of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	$ 321,122	$ 304,690
Gross Unrealized Gains (Losses)	(39)	(5)
Recorded Basis	321,083	304,685
Investments Current [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	52,817	56,175
Gross Unrealized Gains (Losses)	1
Recorded Basis	52,818	56,175
Investments Current [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	267	1,943
Recorded Basis	267	1,943
Investments Current [Member] | Cash and Cash Equivalents [Member] | U.S. Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	500
Recorded Basis	500
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	456	713
Gross Unrealized Gains (Losses)	1	1
Recorded Basis	457	714
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	516	225
Gross Unrealized Gains (Losses)		(1)
Recorded Basis	516	224
Investments Current [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	51,078	48,557
Recorded Basis	51,078	48,557
Investments Noncurrent [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	268,305	248,515
Gross Unrealized Gains (Losses)	(40)	(5)
Recorded Basis	268,265	248,510
Investments Noncurrent [Member] | Investments [Member] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	66	76
Recorded Basis	66	76
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	268	567
Gross Unrealized Gains (Losses)	(1)	(1)
Recorded Basis	267	566
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	28,352	236,036
Recorded Basis	28,352	236,036
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	87,743	834
Gross Unrealized Gains (Losses)	5
Recorded Basis	87,748	834
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] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	1,615	2,069
Gross Unrealized Gains (Losses)		(6)
Recorded Basis	1,615	2,063
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	53,743	2,886
Gross Unrealized Gains (Losses)	(45)	2
Recorded Basis	53,698	$ 2,888
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	56,116
Recorded Basis	56,116
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	40,402
Gross Unrealized Gains (Losses)	1
Recorded Basis	$ 40,403