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Marketable Securities - Schedule of Fair Value and Unrealized Losses of Debt Securities (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Fair Value - Less than 12 Months $ 2,932.9  
Unrealized Loss - Less than 12 months (7.0)  
Fair Value - 12 Months or Greater 0.0  
Unrealized Loss - 12 Months or Greater 0.0  
Fair Value - Total 2,932.9  
Unrealized Loss - Total (7.0)  
Corporate bonds    
Debt Securities, Available-for-sale [Line Items]    
Fair Value - Less than 12 Months 2,905.9 $ 364.5
Unrealized Loss - Less than 12 months (6.9) (0.2)
Fair Value - 12 Months or Greater 0.0 0.0
Unrealized Loss - 12 Months or Greater 0.0 0.0
Fair Value - Total 2,905.9 364.5
Unrealized Loss - Total (6.9) $ (0.2)
U.S. government and government agency obligations    
Debt Securities, Available-for-sale [Line Items]    
Fair Value - Less than 12 Months 27.0  
Unrealized Loss - Less than 12 months (0.1)  
Fair Value - 12 Months or Greater 0.0  
Unrealized Loss - 12 Months or Greater 0.0  
Fair Value - Total 27.0  
Unrealized Loss - Total $ (0.1)