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Debt Securities - Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value $ 405.8 $ 95.0
Less Than 12 Months, Unrealized Loss (0.4) (0.1)
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value 298.5 0.0
Less Than 12 Months, Unrealized Loss (0.3) 0.0
Securities of government-sponsored entities    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value 107.3 95.0
Less Than 12 Months, Unrealized Loss $ (0.1) $ (0.1)