Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	$ 392,615	$ 304,690
Gross Unrealized Gains (Losses)	(1,936)	(5)
Recorded Basis	390,679	304,685
Investments Current [Member] | Cash and Cash Equivalents [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	221	1,943
Recorded Basis	221	1,943
Investments Current [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	121	1,943
Recorded Basis	121	1,943
Investments Current [Member] | Cash and Cash Equivalents [Member] | U.S. Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	100
Recorded Basis	100
Investments Current [Member] | Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	674	5,675
Recorded Basis	674	5,675
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	199
Recorded Basis	199
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	400	225
Gross Unrealized Gains (Losses)		(1)
Recorded Basis	400	224
Investments Current [Member] | Investments [Member] | U.S. Government Agency Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	65
Recorded Basis	65
Investments Current [Member] | Investments [Member] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	10	713
Gross Unrealized Gains (Losses)		1
Recorded Basis	10	714
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] | Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	1,208	643
Gross Unrealized Gains (Losses)	(9)	(1)
Recorded Basis	1,199	642
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	202
Gross Unrealized Gains (Losses)	(2)
Recorded Basis	200
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	419	567
Gross Unrealized Gains (Losses)	(2)	(1)
Recorded Basis	417	566
Investments Noncurrent [Member] | Investments [Member] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost		76
Recorded Basis		76
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	577
Gross Unrealized Gains (Losses)	(5)
Recorded Basis	572
Investments Noncurrent [Member] | Investments [Member] | Asset Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	10
Recorded Basis	10
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	390,512	296,429
Gross Unrealized Gains (Losses)	(1,927)	(4)
Recorded Basis	388,585	296,425
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	16,018	284,593
Recorded Basis	16,018	284,593
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Treasuries [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	45,833	834
Gross Unrealized Gains (Losses)	(143)
Recorded Basis	45,690	834
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Corporate Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	184,808	2,886
Gross Unrealized Gains (Losses)	(953)	2
Recorded Basis	183,855	2,888
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	38,168
Gross Unrealized Gains (Losses)	(222)
Recorded Basis	37,946
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Municipal Bonds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	472	2,069
Gross Unrealized Gains (Losses)	(14)	(6)
Recorded Basis	458	2,063
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] | Mortgage Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	86,240
Gross Unrealized Gains (Losses)	(595)
Recorded Basis	85,645
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Commercial Paper [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cost	18,973
Recorded Basis	$ 18,973