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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | $ 418,685 | $ 363,671
Gross Unrealized Gains (Losses) | 4,783 | 10,450
Recorded Basis | 423,468 | 374,121
Investments Current [Member] | Cash and Cash Equivalents [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 10,742 | 42,007
Recorded Basis | 10,742 | 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 | 10,742 | 42,007
Recorded Basis | 10,742 | 42,007
Investments Current [Member] | Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 102,138 | 99,729
Gross Unrealized Gains (Losses) | 571 | 1,515
Recorded Basis | 102,709 | 101,244
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 42,007 | 50,918
Gross Unrealized Gains (Losses) | 344 | 884
Recorded Basis | 42,351 | 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 | 33,548 | 36,948
Gross Unrealized Gains (Losses) | (25) | (146)
Recorded Basis | 33,523 | 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 | 7,386 | 7,396
Gross Unrealized Gains (Losses) | 548 | 752
Recorded Basis | 7,934 | 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
Gross Unrealized Gains (Losses) |  | 1
Recorded Basis |  | 101
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 19,197 | 4,367
Gross Unrealized Gains (Losses) | (296) | 24
Recorded Basis | 18,901 | 4,391
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 305,805 | 221,935
Gross Unrealized Gains (Losses) | 4,212 | 8,935
Recorded Basis | 310,017 | 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 | 143 | 40,063
Gross Unrealized Gains (Losses) |  | (4)
Recorded Basis | 143 | 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 | 127,026 | 88,902
Gross Unrealized Gains (Losses) | 1,209 | 4,091
Recorded Basis | 128,235 | 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 | 150 | 256
Gross Unrealized Gains (Losses) | 1 | 4
Recorded Basis | 151 | 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 | 32,928 | 29,737
Gross Unrealized Gains (Losses) | 1,703 | 2,466
Recorded Basis | 34,631 | 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 | 67,908 | 4,371
Gross Unrealized Gains (Losses) | (32) | 19
Recorded Basis | 67,876 | 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 | 71,546 | 48,795
Gross Unrealized Gains (Losses) | 1,331 | 2,356
Recorded Basis | 72,877 | 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 | 6,104 | 5,036
Recorded Basis | $ 6,104 | 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

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