Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	$ 561.5	$ 508.6
Short-term investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	460.5	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	147.4	170.2
Short-term investments | Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	69.9	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	7.5	7.5
Short-term investments | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	235.7	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	3.0	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	10.9	10.9
Long-term investments | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	265.4	236.4
Investments Including Cash Equivalents And Restricted Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	741.3	767.8
Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	179.8	259.2
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	0.0	0.1
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	147.4	170.2
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	147.4	170.2
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	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	3.0
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	151.9	174.7
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	151.9	174.6
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	561.5	508.5
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	313.1	345.8
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 | Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of assets on recurring basis	69.9	159.9
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	7.5
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	235.7	178.4
Significant Other Observable Inputs (Level 2) | 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	0.0	0.0
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	10.9
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	265.4	236.4
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	589.4	593.1
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	27.9	84.6
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 | 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 | 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) | 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