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Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | $ 508.6 | $ 267.6
Short-term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 516.0 | 307.3
Short-term investments | Cash and money market fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 170.2 | 73.6
Short-term investments | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value |  | 1.0
Short-term investments | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 159.9 | 49.2
Short-term investments | Domestic and foreign government agencies debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 7.5 | 5.4
Short-term investments | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 178.4 | 178.1
Long-term investments | Cash and money market fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 1.5 |
Long-term investments | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 3.0 | 4.9
Long-term investments | Domestic and foreign government agencies debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 10.9 | 7.5
Long-term investments | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 236.4 | 36.0
Investments Including Cash Equivalents And Restricted Cash |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 767.8 | 355.7
Cash and Cash Equivalents |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Carrying Value | 259.2 | 88.1
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.1 | 1.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 170.2 | 74.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments | Cash and money market fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 170.2 | 73.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis |  | 1.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments | Domestic and foreign government agencies debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term investments | Cash and money market fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 1.5 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term investments | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 3.0 | 4.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term investments | Domestic and foreign government agencies debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term investments | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Investments Including Cash Equivalents And Restricted Cash |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 174.7 | 79.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and Cash Equivalents |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 174.6 | 78.5
Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 508.5 | 266.6
Significant Other Observable Inputs (Level 2) | Short-term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 345.8 | 232.7
Significant Other Observable Inputs (Level 2) | Short-term investments | Cash and money market fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Other Observable Inputs (Level 2) | Short-term investments | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis |  | 0.0
Significant Other Observable Inputs (Level 2) | Short-term investments | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 159.9 | 49.2
Significant Other Observable Inputs (Level 2) | Short-term investments | Domestic and foreign government agencies debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 7.5 | 5.4
Significant Other Observable Inputs (Level 2) | Short-term investments | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 178.4 | 178.1
Significant Other Observable Inputs (Level 2) | Long-term investments | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Other Observable Inputs (Level 2) | Long-term investments | Domestic and foreign government agencies debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 10.9 | 7.5
Significant Other Observable Inputs (Level 2) | Long-term investments | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 236.4 | 36.0
Significant Other Observable Inputs (Level 2) | Investments Including Cash Equivalents And Restricted Cash |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 593.1 | 276.2
Significant Other Observable Inputs (Level 2) | Cash and Cash Equivalents |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 84.6 | 9.6
Significant Unobservable Inputs (Level 3) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Short-term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Short-term investments | Cash and money market fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Short-term investments | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis |  | 0.0
Significant Unobservable Inputs (Level 3) | Short-term investments | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Short-term investments | Domestic and foreign government agencies debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Short-term investments | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Long-term investments | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Long-term investments | Domestic and foreign government agencies debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Long-term investments | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Investments Including Cash Equivalents And Restricted Cash |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Cash and Cash Equivalents |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | $ 0.0 | $ 0.0

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