Quarterly report pursuant to Section 13 or 15(d)

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

v3.19.3
Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 350,141 $ 372,953
Gross Unrealized Gains (Losses) 3,617 (7,005)
Recorded Basis 353,758 365,948
Investments Current [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 6,121 30
Recorded Basis 6,121 30
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 6,121 30
Recorded Basis 6,121 30
Investments Current [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 101,434 417
Gross Unrealized Gains (Losses) 314 (1)
Recorded Basis 101,748 416
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 70,922 20
Gross Unrealized Gains (Losses) 36  
Recorded Basis 70,958 20
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 18,642 347
Gross Unrealized Gains (Losses) (27)  
Recorded Basis 18,615 347
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 8,910 50
Gross Unrealized Gains (Losses) 313 (1)
Recorded Basis 9,223 49
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 2,943  
Gross Unrealized Gains (Losses) (8)  
Recorded Basis 2,935  
Investments Current [Member] | Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 17  
Recorded Basis 17  
Investments Noncurrent [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost   1,710
Gross Unrealized Gains (Losses)   (23)
Recorded Basis   1,687
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost   422
Gross Unrealized Gains (Losses)   (7)
Recorded Basis   415
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost   794
Gross Unrealized Gains (Losses)   (3)
Recorded Basis   791
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost   484
Gross Unrealized Gains (Losses)   (13)
Recorded Basis   471
Investments Noncurrent [Member] | Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost   10
Recorded Basis   10
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 242,586 370,796
Gross Unrealized Gains (Losses) 3,303 (6,981)
Recorded Basis 245,889 363,815
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 19,085 419
Gross Unrealized Gains (Losses) (2)  
Recorded Basis 19,083 419
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 101,533 185,116
Gross Unrealized Gains (Losses) 1,793 (3,739)
Recorded Basis 103,326 181,377
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 18,397 44,304
Gross Unrealized Gains (Losses) 24 (283)
Recorded Basis 18,421 44,021
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 29,780 45,548
Gross Unrealized Gains (Losses) 947 (908)
Recorded Basis 30,727 44,640
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 66,012 89,426
Gross Unrealized Gains (Losses) 507 (2,026)
Recorded Basis 66,519 87,400
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 116 75
Gross Unrealized Gains (Losses) 1 (1)
Recorded Basis 117 74
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Mutual Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 2,844 1,093
Recorded Basis 2,844 1,093
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 4,769 4,765
Gross Unrealized Gains (Losses) 33 (24)
Recorded Basis 4,802 4,741
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 50 50
Recorded Basis $ 50 $ 50