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Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions | Mar. 31, 2020 | Dec. 31, 2019
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | $ 1,050.2 | $ 1,029.3
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 | 190.2 | 115.5
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 | 820.6 | 857.9
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 | 39.4 | 55.9
Cash and money market fund |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 187.0 | 112.3
Cash and money market fund | 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 | 187.0 | 112.3
Total cash and cash equivalents |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 187.0 | 112.3
Total cash and cash equivalents | 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 | 187.0 | 112.3
Certificates of Deposit |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 3.2 | 3.2
Certificates of Deposit | 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 | 3.2 | 3.2
Total restricted cash |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 3.2 | 3.2
Total restricted cash | 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 | 3.2 | 3.2
Commercial paper |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 129.7 | 144.5
Commercial paper | 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 | 129.7 | 144.5
Corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 534.0 | 521.9
Corporate debt securities | 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 | 534.0 | 521.9
Securities of government-sponsored entities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 156.9 | 191.5
Securities of government-sponsored entities | 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 | 156.9 | 191.5
Total debt securities available-for-sale |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 820.6 | 857.9
Total debt securities available-for-sale | 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 | 820.6 | 857.9
Equity securities–biotechnology industry |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 39.4 | 55.9
Equity securities–biotechnology industry | 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 | 39.4 | 55.9
Total equity securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 39.4 | 55.9
Total equity securities | 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 | $ 39.4 | $ 55.9

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