Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	$ 684.4	$ 725.2
Short-term investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	524.1	650.9
Short-term investments | Cash and money market fund
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	72.8	141.7
Short-term investments | Commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	97.5	94.6
Short-term investments | Securities of government-sponsored entities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	45.8	20.8
Short-term investments | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	308.0	393.8
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	4.0
Long-term investments | Securities of government-sponsored entities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	43.1	64.8
Long-term investments | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	133.6	151.2
Long-term investments | Restricted equity securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	56.4
Investments Including Cash Equivalents And Restricted Cash
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	762.7	872.4
Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value	78.3	147.2
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	0.0
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	72.8	141.7
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	72.8	141.7
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 | Securities of government-sponsored entities | 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	4.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term investments | Securities of government-sponsored entities | 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) | Long-term investments | Restricted equity 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
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	78.3	147.2
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	78.3	147.2
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	628.0	725.2
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	451.3	509.2
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	97.5	94.6
Significant Other Observable Inputs (Level 2) | Short-term investments | Securities of government-sponsored entities | Fair Value Measurements Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of assets on recurring basis	45.8	20.8
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	308.0	393.8
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 | Securities of government-sponsored entities | Fair Value Measurements Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of assets on recurring basis	43.1	64.8
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	133.6	151.2
Significant Other Observable Inputs (Level 2) | Long-term investments | Restricted equity 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
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	628.0	725.2
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	0.0
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	56.4	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 | Securities of government-sponsored entities | 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 | Securities of government-sponsored entities | 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) | Long-term investments | Restricted equity 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	56.4	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	56.4	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