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Investments (Detail) - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Schedule Of Investments [Line Items] |  |
Investments | $ 48,915 |
Investments | 757,233 | $ 725,227
Commercial paper |  |
Schedule Of Investments [Line Items] |  |
Investments | 135,197 | 94,572
Corporate debt securities |  |
Schedule Of Investments [Line Items] |  |
Investments | 414,951 | 544,978
Securities of government-sponsored entities |  |
Schedule Of Investments [Line Items] |  |
Investments | 158,170 | $ 85,677
Restricted equity securities |  |
Schedule Of Investments [Line Items] |  |
Investments | $ 48,915 |

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