Quarterly report pursuant to Section 13 or 15(d)

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

v3.21.1
Fair Value Measurement - Summary of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis $ 445,638 $ 374,121
Other [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 39,519 82,066
Level 1 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 5,821 5,036
Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 400,298 287,019
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 225 42,007
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 225 42,007
Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 59,104 51,802
Investments [Member] | Corporate Bonds [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 59,104 51,802
Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 9,907 4,391
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 9,907 4,391
Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 100 101
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 100 101
Investments [Member] | U.S. Government Agency Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 8,031 8,148
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 8,031 8,148
Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 37,000 36,802
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 37,000 36,802
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 39,294 40,059
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 39,294 40,059
Restricted Cash and Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 149,737 92,993
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 149,737 92,993
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 70,293 51,151
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 70,293 51,151
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 29,232 4,390
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 29,232 4,390
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 36,654 32,203
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 36,654 32,203
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 240 260
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 240 260
Restricted Cash and Investments [Member] | Mutual Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis 5,821 5,036
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 $ 5,821 5,036
Restricted Cash and Investments [Member] | Supranational Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total Recorded Basis   4,778
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   $ 4,778