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
Jun. 30, 2020
Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 386,467 $ 361,468
Gross Unrealized Gains (Losses) 9,622 3,896
Recorded Basis 396,089 365,364
Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 58,299 35,526
Recorded Basis 58,299 35,526
Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 58,299 35,526
Recorded Basis 58,299 35,526
Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 67,257 82,155
Gross Unrealized Gains (Losses) 216 435
Recorded Basis 67,473 82,590
Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 20,167 53,493
Gross Unrealized Gains (Losses) 499 156
Recorded Basis 20,666 53,649
Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 36,961 14,017
Gross Unrealized Gains (Losses) (1,001) (13)
Recorded Basis 35,960 14,004
Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 7,402 7,408
Gross Unrealized Gains (Losses) 702 282
Recorded Basis 8,104 7,690
Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 2,400 4,500
Gross Unrealized Gains (Losses) 5 9
Recorded Basis 2,405 4,509
Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 327 2,737
Gross Unrealized Gains (Losses) 11 1
Recorded Basis 338 2,738
Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 260,911 243,787
Gross Unrealized Gains (Losses) 9,406 3,461
Recorded Basis 270,317 247,248
Restricted Cash and Investments [Member] | Money Market Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 32,864 29,046
Gross Unrealized Gains (Losses) 10  
Recorded Basis 32,874 29,046
Restricted Cash and Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 95,827 98,481
Gross Unrealized Gains (Losses) 4,506 1,931
Recorded Basis 100,333 100,412
Restricted Cash and Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 280 303
Gross Unrealized Gains (Losses) 5  
Recorded Basis 285 303
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 25,718 27,885
Gross Unrealized Gains (Losses) 2,394 642
Recorded Basis 28,112 28,527
Restricted Cash and Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 6,320 16,906
Gross Unrealized Gains (Losses) 46 21
Recorded Basis 6,366 16,927
Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 56,551 62,930
Gross Unrealized Gains (Losses) 2,356 837
Recorded Basis 58,907 63,767
Restricted Cash and Investments [Member] | Commercial Paper [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 31,844  
Gross Unrealized Gains (Losses) 26  
Recorded Basis 31,870  
Restricted Cash and Investments [Member] | Municipal Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 2,473  
Gross Unrealized Gains (Losses) 7  
Recorded Basis 2,480  
Restricted Cash and Investments [Member] | Supranational Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 4,772 4,770
Gross Unrealized Gains (Losses) 56 30
Recorded Basis 4,828 4,800
Restricted Cash and Investments [Member] | Mutual Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 4,262 3,466
Recorded Basis $ 4,262 $ 3,466