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Fair Value Measurement - Summary of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | $ 370,622 | $ 365,364
Other [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 65,605 | 64,572
Level 1 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 4,712 | 3,466
Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 300,305 | 297,326
Cash and Cash Equivalents [Member] | Money Market Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 32,723 | 35,526
Cash and Cash Equivalents [Member] | Money Market Funds [Member] | Other [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total 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] |  |
Total Recorded Basis | 9,999 |
Cash and Cash Equivalents [Member] | Discount Notes [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 9,999 |
Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 37,290 | 53,649
Investments [Member] | Corporate Bonds [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 37,290 | 53,649
Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 4,478 | 2,738
Investments [Member] | Mortgage Backed Securities [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 4,478 | 2,738
Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 101 | 4,509
Investments [Member] | U.S. Treasuries [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 101 | 4,509
Investments [Member] | U.S. Government Agency Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 8,190 | 7,690
Investments [Member] | U.S. Government Agency Securities [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 8,190 | 7,690
Investments [Member] | Asset Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 36,620 | 14,004
Investments [Member] | Asset Backed Securities [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 36,620 | 14,004
Restricted Cash and Investments [Member] | Money Market Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 32,882 | 29,046
Restricted Cash and Investments [Member] | Money Market Funds [Member] | Other [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total 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] |  |
Total Recorded Basis | 100,342 | 100,412
Restricted Cash and Investments [Member] | Corporate Bonds [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 100,342 | 100,412
Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 55,131 | 63,767
Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 55,131 | 63,767
Restricted Cash and Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 4,791 | 16,927
Restricted Cash and Investments [Member] | U.S. Treasuries [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 4,791 | 16,927
Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 28,211 | 28,527
Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 28,211 | 28,527
Restricted Cash and Investments [Member] | Asset Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 275 | 303
Restricted Cash and Investments [Member] | Asset Backed Securities [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 275 | 303
Restricted Cash and Investments [Member] | Commercial Paper [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 5,999 |
Restricted Cash and Investments [Member] | Commercial Paper [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 5,999 |
Restricted Cash and Investments [Member] | Mutual Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 4,712 | 3,466
Restricted Cash and Investments [Member] | Mutual Funds [Member] | Level 1 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 4,712 | 3,466
Restricted Cash and Investments [Member] | Supranational Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 4,802 | 4,800
Restricted Cash and Investments [Member] | Supranational Bonds [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 4,802 | $ 4,800
Restricted Cash and Investments [Member] | Municipal Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | 4,076 |
Restricted Cash and Investments [Member] | Municipal Bonds [Member] | Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total Recorded Basis | $ 4,076 |

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