Quarterly report pursuant to Section 13 or 15(d)

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

v3.20.2
Fair Value Measurement - Summary of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis $ 396,089 $ 365,364
Other [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 91,173 64,572
Level 1 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 4,262 3,466
Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 300,654 297,326
Cash and Cash Equivalents [Member] | Money Market Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 58,299 35,526
Cash and Cash Equivalents [Member] | Money Market Funds [Member] | Other [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 58,299 35,526
Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 20,666 53,649
Investments [Member] | Corporate Bonds [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 20,666 53,649
Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 338 2,738
Investments [Member] | Mortgage Backed Securities [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 338 2,738
Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 2,405 4,509
Investments [Member] | U.S. Treasuries [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 2,405 4,509
Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 8,104 7,690
Investments [Member] | U.S. Government Agency Securities [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 8,104 7,690
Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 35,960 14,004
Investments [Member] | Asset Backed Securities [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 35,960 14,004
Restricted Cash and Investments [Member] | Money Market Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 32,874 29,046
Restricted Cash and Investments [Member] | Money Market Funds [Member] | Other [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total 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]    
Total Recorded Basis 100,333 100,412
Restricted Cash and Investments [Member] | Corporate Bonds [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 100,333 100,412
Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 58,907 63,767
Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 58,907 63,767
Restricted Cash and Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 6,366 16,927
Restricted Cash and Investments [Member] | U.S. Treasuries [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 6,366 16,927
Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 28,112 28,527
Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 28,112 28,527
Restricted Cash and Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 285 303
Restricted Cash and Investments [Member] | Asset Backed Securities [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 285 303
Restricted Cash and Investments [Member] | Commercial Paper [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 31,870  
Restricted Cash and Investments [Member] | Commercial Paper [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 31,870  
Restricted Cash and Investments [Member] | Mutual Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 4,262 3,466
Restricted Cash and Investments [Member] | Mutual Funds [Member] | Level 1 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 4,262 3,466
Restricted Cash and Investments [Member] | Supranational Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 4,828 4,800
Restricted Cash and Investments [Member] | Supranational Bonds [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 4,828 $ 4,800
Restricted Cash and Investments [Member] | Municipal Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 2,480  
Restricted Cash and Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis $ 2,480