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
Mar. 31, 2021
Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost $ 442,445 $ 363,671
Gross Unrealized Gains (Losses) 3,193 10,450
Recorded Basis 445,638 374,121
Investments Current [Member] | Cash and Cash Equivalents [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 225 42,007
Recorded Basis 225 42,007
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 225 42,007
Recorded Basis 225 42,007
Investments Current [Member] | Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 113,550 99,729
Gross Unrealized Gains (Losses) 592 1,515
Recorded Basis 114,142 101,244
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 59,034 50,918
Gross Unrealized Gains (Losses) 70 884
Recorded Basis 59,104 51,802
Investments Current [Member] | Investments [Member] | Asset Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 36,945 36,948
Gross Unrealized Gains (Losses) 55 (146)
Recorded Basis 37,000 36,802
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 7,393 7,396
Gross Unrealized Gains (Losses) 638 752
Recorded Basis 8,031 8,148
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 100 100
Gross Unrealized Gains (Losses)   1
Recorded Basis 100 101
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 10,078 4,367
Gross Unrealized Gains (Losses) (171) 24
Recorded Basis 9,907 4,391
Investments Noncurrent [Member] | Restricted Cash and Investments [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cost 328,670 221,935
Gross Unrealized Gains (Losses) 2,601 8,935
Recorded Basis 331,271 230,870
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 39,290 40,063
Gross Unrealized Gains (Losses) 4 (4)
Recorded Basis 39,294 40,059
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 149,582 88,902
Gross Unrealized Gains (Losses) 155 4,091
Recorded Basis 149,737 92,993
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 237 256
Gross Unrealized Gains (Losses) 3 4
Recorded Basis 240 260
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 34,937 29,737
Gross Unrealized Gains (Losses) 1,717 2,466
Recorded Basis 36,654 32,203
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 30,129 4,371
Gross Unrealized Gains (Losses) (897) 19
Recorded Basis 29,232 4,390
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 68,674 48,795
Gross Unrealized Gains (Losses) 1,619 2,356
Recorded Basis 70,293 51,151
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 5,821 5,036
Recorded Basis $ 5,821 5,036
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   4,775
Gross Unrealized Gains (Losses)   3
Recorded Basis   $ 4,778