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Available-For-Sale Debt Securities (Detail) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Schedule Of Investments [Line Items] |  |
Marketable securities | $ 857,899 | $ 725,227
Commercial paper |  |
Schedule Of Investments [Line Items] |  |
Marketable securities | 144,472 | 94,572
Corporate debt securities |  |
Schedule Of Investments [Line Items] |  |
Marketable securities | 521,946 | 544,978
Securities of government-sponsored entities |  |
Schedule Of Investments [Line Items] |  |
Marketable securities | $ 191,481 | $ 85,677

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