Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurement - Summary of Investments (Detail)

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Fair Value Measurement - Summary of Investments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost $ 368,201 $ 304,690
Gross Unrealized Gains (Losses) 311 (5)
Recorded Basis 368,512 304,685
Investments Current [Member] | Cash and Cash Equivalents [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 104 1,943
Recorded Basis 104 1,943
Investments Current [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 54 1,943
Recorded Basis 54 1,943
Investments Current [Member] | Cash and Cash Equivalents [Member] | U.S. Treasuries [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 50  
Recorded Basis 50  
Investments Current [Member] | Investments [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 796 5,675
Recorded Basis 796 5,675
Investments Current [Member] | Investments [Member] | Municipal Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 70 713
Gross Unrealized Gains (Losses)   1
Recorded Basis 70 714
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 565 225
Gross Unrealized Gains (Losses)   (1)
Recorded Basis 565 224
Investments Current [Member] | Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 161  
Recorded Basis 161  
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 1,192 643
Gross Unrealized Gains (Losses) (1) (1)
Recorded Basis 1,191 642
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 203  
Gross Unrealized Gains (Losses) (1)  
Recorded Basis 202  
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] | Corporate Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 330 567
Gross Unrealized Gains (Losses)   (1)
Recorded Basis 330 566
Investments Noncurrent [Member] | Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 5  
Recorded Basis 5  
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 654  
Recorded Basis 654  
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 366,109 296,429
Gross Unrealized Gains (Losses) 312 (4)
Recorded Basis 366,421 296,425
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 17,600 284,593
Recorded Basis 17,600 284,593
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Treasuries [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 27,660 834
Gross Unrealized Gains (Losses) (27)  
Recorded Basis 27,633 834
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Municipal Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 736 2,069
Gross Unrealized Gains (Losses) (8) (6)
Recorded Basis 728 2,063
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Corporate Bonds [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 162,500 2,886
Gross Unrealized Gains (Losses) 214 2
Recorded Basis 162,714 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 35,314  
Gross Unrealized Gains (Losses) (15)  
Recorded Basis 35,299  
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 84,859  
Gross Unrealized Gains (Losses) 148  
Recorded Basis 85,007  
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Commercial Paper [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cost 37,440  
Recorded Basis $ 37,440