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Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | $ 591.7 | $ 508.6
Short-term investments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 508.7 | 516.0
Short-term investments | Cash and money market fund |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 166.4 | 170.2
Short-term investments | Commercial paper |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 37.1 | 159.9
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 | 6.0 | 7.5
Short-term investments | Corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 299.2 | 178.4
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 | 1.5
Long-term investments | Certificates of deposit |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 4.0 | 3.0
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 | 5.0 | 10.9
Long-term investments | Corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 244.4 | 236.4
Investments Including Cash Equivalents And Restricted Cash |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 763.6 | 767.8
Cash and Cash Equivalents |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Carrying Value | 171.9 | 259.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.1
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 166.4 | 170.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments | Cash and money market fund | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 166.4 | 170.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments | Commercial paper | Fair Value Measurements Recurring |  |
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 Measurements Recurring |  |
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 Measurements Recurring |  |
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 Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 1.5 | 1.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term investments | Certificates of deposit | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 4.0 | 3.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term investments | Domestic and foreign government agencies debt securities | Fair Value Measurements Recurring |  |
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 Measurements Recurring |  |
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 Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 171.9 | 174.7
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and Cash Equivalents | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 171.9 | 174.6
Significant Other Observable Inputs (Level 2) | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 591.7 | 508.5
Significant Other Observable Inputs (Level 2) | Short-term investments | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 342.3 | 345.8
Significant Other Observable Inputs (Level 2) | Short-term investments | Cash and money market fund | Fair Value Measurements Recurring |  |
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 | Commercial paper | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 37.1 | 159.9
Significant Other Observable Inputs (Level 2) | Short-term investments | Domestic and foreign government agencies debt securities | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 6.0 | 7.5
Significant Other Observable Inputs (Level 2) | Short-term investments | Corporate debt securities | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 299.2 | 178.4
Significant Other Observable Inputs (Level 2) | Long-term investments | Cash and money market fund | Fair Value Measurements Recurring |  |
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 | Certificates of deposit | Fair Value Measurements Recurring |  |
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 Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 5.0 | 10.9
Significant Other Observable Inputs (Level 2) | Long-term investments | Corporate debt securities | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 244.4 | 236.4
Significant Other Observable Inputs (Level 2) | Investments Including Cash Equivalents And Restricted Cash | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 591.7 | 593.1
Significant Other Observable Inputs (Level 2) | Cash and Cash Equivalents | Fair Value Measurements Recurring |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 84.6
Significant Unobservable Inputs (Level 3) | Fair Value Measurements Recurring |  |
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 Measurements Recurring |  |
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 Measurements Recurring |  |
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 | Commercial paper | Fair Value Measurements Recurring |  |
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 Measurements Recurring |  |
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 Measurements Recurring |  |
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 | Cash and money market fund | Fair Value Measurements Recurring |  |
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 Measurements Recurring |  |
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 Measurements Recurring |  |
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 Measurements Recurring |  |
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 Measurements Recurring |  |
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 Measurements Recurring |  |
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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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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