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Fair Value Measurements - Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Millions
Mar. 31, 2022
Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 1,305.0 $ 1,338.9
Level 1 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets on recurring basis 369.3 396.7
Level 2 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets on recurring basis 935.7 931.2
Level 3 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets on recurring basis 0.0 11.0
Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 499.2 485.5
Corporate debt securities | Level 1 | Fair Value, 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
Corporate debt securities | Level 2 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets on recurring basis 499.2 485.5
Corporate debt securities | Level 3 | Fair Value, 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
Securities of government-sponsored entities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 337.8 240.9
Securities of government-sponsored entities | Level 1 | Fair Value, 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
Securities of government-sponsored entities | Level 2 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets on recurring basis 337.8 240.9
Securities of government-sponsored entities | Level 3 | Fair Value, 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
Equity securities–biotechnology industry    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 91.3 63.7
Equity securities–biotechnology industry | Level 1 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets on recurring basis 91.3 52.7
Equity securities–biotechnology industry | Level 2 | Fair Value, 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
Equity securities–biotechnology industry | Level 3 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets on recurring basis 0.0 11.0
Cash and money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 270.2 340.8
Cash and money market funds | Level 1 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets on recurring basis 270.2 340.8
Cash and money market funds | Level 2 | Fair Value, 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
Cash and money market funds | Level 3 | Fair Value, 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
Certificates of deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 7.8 3.2
Certificates of deposit | Level 1 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets on recurring basis 7.8 3.2
Certificates of deposit | Level 2 | Fair Value, 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
Certificates of deposit | Level 3 | Fair Value, 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
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 98.7 204.8
Commercial paper | Level 1 | Fair Value, 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
Commercial paper | Level 2 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets on recurring basis 98.7 204.8
Commercial paper | Level 3 | Fair Value, 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