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Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale investments | $ 1,996,321 | $ 347,326
Cash | 2,752 | 4,460
Total cash and investments in marketable securities | 2,106,085 | 353,220
Corporate Commercial Paper [Member] | Fair Value Hierarchy Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale investments | 1,201,026 | 128,096
Corporate Notes and Bonds [Member] | Fair Value Hierarchy Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale investments | 671,323 | 216,253
Obligations of U.S. government agencies [Member] | Fair Value Hierarchy Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale investments | 123,972 | 2,977
Money Market Funds [Member] | Fair Value Hierarchy Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in marketable securities | 100,299 | 302
Certificates of Deposit [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in marketable securities | $ 6,713 | $ 1,132

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