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, 2020
Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 360,049 $ 361,468
Gross Unrealized Gains (Losses) 10,573 3,896
Recorded Basis 370,622 365,364
Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 42,721 35,526
Gross Unrealized Gains (Losses) 1  
Recorded Basis 42,722 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 32,723 35,526
Recorded Basis 32,723 35,526
Cash and Cash Equivalents [Member] | Discount Notes [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 9,998  
Gross Unrealized Gains (Losses) 1  
Recorded Basis 9,999  
Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 85,646 82,155
Gross Unrealized Gains (Losses) 1,033 435
Recorded Basis 86,679 82,590
Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 36,732 53,493
Gross Unrealized Gains (Losses) 558 156
Recorded Basis 37,290 53,649
Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 36,950 14,017
Gross Unrealized Gains (Losses) (330) (13)
Recorded Basis 36,620 14,004
Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 7,399 7,408
Gross Unrealized Gains (Losses) 791 282
Recorded Basis 8,190 7,690
Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 100 4,500
Gross Unrealized Gains (Losses) 1 9
Recorded Basis 101 4,509
Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 4,465 2,737
Gross Unrealized Gains (Losses) 13 1
Recorded Basis 4,478 2,738
Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 231,682 243,787
Gross Unrealized Gains (Losses) 9,539 3,461
Recorded Basis 241,221 247,248
Restricted Cash and Investments [Member] | Money Market Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 32,879 29,046
Gross Unrealized Gains (Losses) 3  
Recorded Basis 32,882 29,046
Restricted Cash and Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 95,881 98,481
Gross Unrealized Gains (Losses) 4,461 1,931
Recorded Basis 100,342 100,412
Restricted Cash and Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 270 303
Gross Unrealized Gains (Losses) 5  
Recorded Basis 275 303
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 25,697 27,885
Gross Unrealized Gains (Losses) 2,514 642
Recorded Basis 28,211 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 4,760 16,906
Gross Unrealized Gains (Losses) 31 21
Recorded Basis 4,791 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 52,650 62,930
Gross Unrealized Gains (Losses) 2,481 837
Recorded Basis 55,131 63,767
Restricted Cash and Investments [Member] | Commercial Paper [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 5,998  
Gross Unrealized Gains (Losses) 1  
Recorded Basis 5,999  
Restricted Cash and Investments [Member] | Municipal Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 4,062  
Gross Unrealized Gains (Losses) 14  
Recorded Basis 4,076  
Restricted Cash and Investments [Member] | Supranational Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 4,773 4,770
Gross Unrealized Gains (Losses) 29 30
Recorded Basis 4,802 4,800
Restricted Cash and Investments [Member] | Mutual Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 4,712 3,466
Recorded Basis $ 4,712 $ 3,466