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Investments (Detail) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Investments [Line Items] |  |
Investments | $ 453,458 | $ 267,573
Certificates of deposit |  |
Schedule of Investments [Line Items] |  |
Investments | 240 | 960
Commercial paper |  |
Schedule of Investments [Line Items] |  |
Investments | 66,735 | 49,245
Corporate debt securities |  |
Schedule of Investments [Line Items] |  |
Investments | 363,012 | 204,436
Domestic and foreign government agencies debt securities |  |
Schedule of Investments [Line Items] |  |
Investments | $ 23,471 | $ 12,932

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