Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail)

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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 387,761 $ 392,615
Gross Unrealized Gains (Losses) (10,237) (1,936)
Recorded Basis 377,524 390,679
Investments Current [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 61 221
Recorded Basis 61 221
Investments Current [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 61 121
Recorded Basis 61 121
Investments Current [Member] | Cash and Cash Equivalents [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost   100
Recorded Basis   100
Investments Current [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 396 674
Gross Unrealized Gains (Losses) (1)  
Recorded Basis 395 674
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 346 199
Gross Unrealized Gains (Losses) (1)  
Recorded Basis 345 199
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 45 400
Recorded Basis 45 400
Investments Current [Member] | Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 5 65
Recorded Basis 5 65
Investments Current [Member] | Investments [Member] | Municipal Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost   10
Recorded Basis   10
Investments Noncurrent [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 1,689 1,208
Gross Unrealized Gains (Losses) (40) (9)
Recorded Basis 1,649 1,199
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 745 202
Gross Unrealized Gains (Losses) (10) (2)
Recorded Basis 735 200
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 383 419
Gross Unrealized Gains (Losses) (11) (2)
Recorded Basis 372 417
Investments Noncurrent [Member] | Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 50  
Gross Unrealized Gains (Losses) (1)  
Recorded Basis 49  
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 501 577
Gross Unrealized Gains (Losses) (18) (5)
Recorded Basis 483 572
Investments Noncurrent [Member] | Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 10 10
Recorded Basis 10 10
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 385,615 390,512
Gross Unrealized Gains (Losses) (10,196) (1,927)
Recorded Basis 375,419 388,585
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 416 16,018
Recorded Basis 416 16,018
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 54,296 45,833
Gross Unrealized Gains (Losses) (492) (143)
Recorded Basis 53,804 45,690
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 187,203 184,808
Gross Unrealized Gains (Losses) (4,673) (953)
Recorded Basis 182,530 183,855
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 45,570 38,168
Gross Unrealized Gains (Losses) (1,785) (222)
Recorded Basis 43,785 37,946
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Municipal Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 80 472
Gross Unrealized Gains (Losses)   (14)
Recorded Basis 80 458
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 93,211 86,240
Gross Unrealized Gains (Losses) (3,178) (595)
Recorded Basis 90,033 85,645
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 75  
Gross Unrealized Gains (Losses) (1)  
Recorded Basis 74  
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Commercial Paper [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost   18,973
Recorded Basis   $ 18,973
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Supranational Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 4,764  
Gross Unrealized Gains (Losses) (67)  
Recorded Basis $ 4,697