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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | $ 360,049 | $ 361,468
Gross Unrealized Gains (Losses) | 10,573 | 3,896
Recorded Basis | 370,622 | 365,364
Cash and Cash Equivalents [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 42,721 | 35,526
Gross Unrealized Gains (Losses) | 1 |
Recorded Basis | 42,722 | 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 | 32,723 | 35,526
Recorded Basis | 32,723 | 35,526
Cash and Cash Equivalents [Member] | Discount Notes [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 9,998 |
Gross Unrealized Gains (Losses) | 1 |
Recorded Basis | 9,999 |
Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 85,646 | 82,155
Gross Unrealized Gains (Losses) | 1,033 | 435
Recorded Basis | 86,679 | 82,590
Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 36,732 | 53,493
Gross Unrealized Gains (Losses) | 558 | 156
Recorded Basis | 37,290 | 53,649
Investments [Member] | Asset Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 36,950 | 14,017
Gross Unrealized Gains (Losses) | (330) | (13)
Recorded Basis | 36,620 | 14,004
Investments [Member] | U.S. Government Agency Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 7,399 | 7,408
Gross Unrealized Gains (Losses) | 791 | 282
Recorded Basis | 8,190 | 7,690
Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 100 | 4,500
Gross Unrealized Gains (Losses) | 1 | 9
Recorded Basis | 101 | 4,509
Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 4,465 | 2,737
Gross Unrealized Gains (Losses) | 13 | 1
Recorded Basis | 4,478 | 2,738
Restricted Cash and Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 231,682 | 243,787
Gross Unrealized Gains (Losses) | 9,539 | 3,461
Recorded Basis | 241,221 | 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 | 32,879 | 29,046
Gross Unrealized Gains (Losses) | 3 |
Recorded Basis | 32,882 | 29,046
Restricted Cash and Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 95,881 | 98,481
Gross Unrealized Gains (Losses) | 4,461 | 1,931
Recorded Basis | 100,342 | 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 | 270 | 303
Gross Unrealized Gains (Losses) | 5 |
Recorded Basis | 275 | 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 | 25,697 | 27,885
Gross Unrealized Gains (Losses) | 2,514 | 642
Recorded Basis | 28,211 | 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 | 4,760 | 16,906
Gross Unrealized Gains (Losses) | 31 | 21
Recorded Basis | 4,791 | 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 | 52,650 | 62,930
Gross Unrealized Gains (Losses) | 2,481 | 837
Recorded Basis | 55,131 | 63,767
Restricted Cash and Investments [Member] | Commercial Paper [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 5,998 |
Gross Unrealized Gains (Losses) | 1 |
Recorded Basis | 5,999 |
Restricted Cash and Investments [Member] | Municipal Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 4,062 |
Gross Unrealized Gains (Losses) | 14 |
Recorded Basis | 4,076 |
Restricted Cash and Investments [Member] | Supranational Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 4,773 | 4,770
Gross Unrealized Gains (Losses) | 29 | 30
Recorded Basis | 4,802 | 4,800
Restricted Cash and Investments [Member] | Mutual Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 4,712 | 3,466
Recorded Basis | $ 4,712 | $ 3,466

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