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Investments (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule Of Investments [Line Items] |  |
Investments | $ 591,706 | $ 508,578
Commercial paper |  |
Schedule Of Investments [Line Items] |  |
Investments | 37,157 | 75,362
Corporate debt securities |  |
Schedule Of Investments [Line Items] |  |
Investments | 543,648 | 414,815
Domestic and foreign government agencies debt securities |  |
Schedule Of Investments [Line Items] |  |
Investments | $ 10,901 | $ 18,401

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