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Debt Securities - Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value $ 245.5  
Less Than 12 Months, Unrealized Loss (0.3)  
12 Months or Longer, Fair Value 0.0  
12 Months or Longer, Unrealized Loss 0.0  
Total Fair Value 245.5  
Total Unrealized Loss (0.3)  
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value 211.0  
Less Than 12 Months, Unrealized Loss (0.2)  
12 Months or Longer, Fair Value 0.0  
12 Months or Longer, Unrealized Loss 0.0  
Total Fair Value 211.0  
Total Unrealized Loss (0.2)  
Securities of government-sponsored entities    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value 34.5 $ 95.0
Less Than 12 Months, Unrealized Loss (0.1) (0.1)
12 Months or Longer, Fair Value 0.0 0.0
12 Months or Longer, Unrealized Loss 0.0 0.0
Total Fair Value 34.5 95.0
Total Unrealized Loss $ (0.1) $ (0.1)