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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | $ 386,467 | $ 361,468
Gross Unrealized Gains (Losses) | 9,622 | 3,896
Recorded Basis | 396,089 | 365,364
Cash and Cash Equivalents [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 58,299 | 35,526
Recorded Basis | 58,299 | 35,526
Cash and Cash Equivalents [Member] | Money Market Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 58,299 | 35,526
Recorded Basis | 58,299 | 35,526
Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 67,257 | 82,155
Gross Unrealized Gains (Losses) | 216 | 435
Recorded Basis | 67,473 | 82,590
Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 20,167 | 53,493
Gross Unrealized Gains (Losses) | 499 | 156
Recorded Basis | 20,666 | 53,649
Investments [Member] | Asset Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 36,961 | 14,017
Gross Unrealized Gains (Losses) | (1,001) | (13)
Recorded Basis | 35,960 | 14,004
Investments [Member] | U.S. Government Agency Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 7,402 | 7,408
Gross Unrealized Gains (Losses) | 702 | 282
Recorded Basis | 8,104 | 7,690
Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 2,400 | 4,500
Gross Unrealized Gains (Losses) | 5 | 9
Recorded Basis | 2,405 | 4,509
Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 327 | 2,737
Gross Unrealized Gains (Losses) | 11 | 1
Recorded Basis | 338 | 2,738
Restricted Cash and Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 260,911 | 243,787
Gross Unrealized Gains (Losses) | 9,406 | 3,461
Recorded Basis | 270,317 | 247,248
Restricted Cash and Investments [Member] | Money Market Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 32,864 | 29,046
Gross Unrealized Gains (Losses) | 10 |
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] |  |
Cost | 95,827 | 98,481
Gross Unrealized Gains (Losses) | 4,506 | 1,931
Recorded Basis | 100,333 | 100,412
Restricted Cash and Investments [Member] | Asset Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 280 | 303
Gross Unrealized Gains (Losses) | 5 |
Recorded Basis | 285 | 303
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 | 25,718 | 27,885
Gross Unrealized Gains (Losses) | 2,394 | 642
Recorded Basis | 28,112 | 28,527
Restricted Cash and Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 6,320 | 16,906
Gross Unrealized Gains (Losses) | 46 | 21
Recorded Basis | 6,366 | 16,927
Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 56,551 | 62,930
Gross Unrealized Gains (Losses) | 2,356 | 837
Recorded Basis | 58,907 | 63,767
Restricted Cash and Investments [Member] | Commercial Paper [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 31,844 |
Gross Unrealized Gains (Losses) | 26 |
Recorded Basis | 31,870 |
Restricted Cash and Investments [Member] | Municipal Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 2,473 |
Gross Unrealized Gains (Losses) | 7 |
Recorded Basis | 2,480 |
Restricted Cash and Investments [Member] | Supranational Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 4,772 | 4,770
Gross Unrealized Gains (Losses) | 56 | 30
Recorded Basis | 4,828 | 4,800
Restricted Cash and Investments [Member] | Mutual Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 4,262 | 3,466
Recorded Basis | $ 4,262 | $ 3,466

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