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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | $ 489,744 | $ 363,671
Gross Unrealized Gains (Losses) | 6,172 | 10,450
Recorded Basis | 495,916 | 374,121
Investments Current [Member] | Cash and Cash Equivalents [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 1,149 | 42,007
Recorded Basis | 1,149 | 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 | 1,149 | 42,007
Recorded Basis | 1,149 | 42,007
Investments Current [Member] | Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 106,268 | 99,729
Gross Unrealized Gains (Losses) | 1,016 | 1,515
Recorded Basis | 107,284 | 101,244
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 48,243 | 50,918
Gross Unrealized Gains (Losses) | 541 | 884
Recorded Basis | 48,784 | 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 | 36,943 | 36,948
Gross Unrealized Gains (Losses) | (7) | (146)
Recorded Basis | 36,936 | 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 | 7,389 | 7,396
Gross Unrealized Gains (Losses) | 593 | 752
Recorded Basis | 7,982 | 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 | 2,101 | 100
Gross Unrealized Gains (Losses) | 14 | 1
Recorded Basis | 2,115 | 101
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 11,592 | 4,367
Gross Unrealized Gains (Losses) | (125) | 24
Recorded Basis | 11,467 | 4,391
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 382,327 | 221,935
Gross Unrealized Gains (Losses) | 5,156 | 8,935
Recorded Basis | 387,483 | 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 | 396 | 40,063
Gross Unrealized Gains (Losses) |  | (4)
Recorded Basis | 396 | 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 | 183,186 | 88,902
Gross Unrealized Gains (Losses) | 1,628 | 4,091
Recorded Basis | 184,814 | 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 | 203 | 256
Gross Unrealized Gains (Losses) | 2 | 4
Recorded Basis | 205 | 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 | 34,908 | 29,737
Gross Unrealized Gains (Losses) | 1,909 | 2,466
Recorded Basis | 36,817 | 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 | 69,225 | 4,371
Gross Unrealized Gains (Losses) | 147 | 19
Recorded Basis | 69,372 | 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 | 88,288 | 48,795
Gross Unrealized Gains (Losses) | 1,470 | 2,356
Recorded Basis | 89,758 | 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 | 6,121 | 5,036
Recorded Basis | $ 6,121 | 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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