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Fair Value Measurements - Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 1,165.4	$ 1,069.5
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	355.8	190.3
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	770.7	841.0
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	38.9	38.2
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	409.4	445.9
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	409.4	445.9
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	268.1	312.9
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	268.1	312.9
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
Total debt securities available-for-sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	770.7	841.0
Total debt securities available-for-sale | 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
Total debt securities available-for-sale | 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	770.7	841.0
Total debt securities available-for-sale | 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	38.9	38.2
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	0.0	0.0
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	38.9	38.2
Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	38.9	38.2
Total equity 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
Total equity 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	0.0	0.0
Total equity 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	38.9	38.2
Cash and money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	352.6	187.1
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	352.6	187.1
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
Total cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	352.6	187.1
Total cash and cash equivalents | 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	352.6	187.1
Total cash and cash equivalents | 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
Total cash and cash equivalents | 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	3.2	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	3.2	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
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, 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 | 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
Total restricted cash | 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	93.2	82.2
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	93.2	82.2
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

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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.

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- Definition

Sum of the carrying amounts as of the balance sheet date of all investments.

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