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Financial Instruments and Fair Value , Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 890.4 $ 986.2
Unrealized Gains 2.0 1.3
Unrealized Losses (0.1) (0.1)
Fair Value 892.3 987.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 187.9 203.4
Less Than 12 Months, Unrealized Losses (0.1) (0.1)
12 Months or Greater, Fair Value 0.0 16.1
12 Months or Greater, Unrealized Losses 0.0 0.0
Total, Fair Value 187.9 219.5
Total, Unrealized Losses (0.1) (0.1)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Due within one year 774.4 843.1
Due within one to three years 117.9 144.3
Fair Value 892.3 987.4
Corporate debt securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 501.1 576.1
Unrealized Gains 1.8 1.0
Unrealized Losses (0.1) (0.1)
Fair Value 502.8 577.0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 103.9 117.3
Less Than 12 Months, Unrealized Losses 0.0 (0.1)
12 Months or Greater, Fair Value 0.0 16.1
12 Months or Greater, Unrealized Losses 0.0 0.0
Total, Fair Value 103.9 133.4
Total, Unrealized Losses 0.0 (0.1)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value 502.8 577.0
Commercial paper [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 125.2 148.7
Unrealized Gains 0.1 0.1
Unrealized Losses 0.0 0.0
Fair Value 125.3 148.8
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 24.6 26.0
Less Than 12 Months, Unrealized Losses 0.0 0.0
12 Months or Greater, Fair Value 0.0 0.0
12 Months or Greater, Unrealized Losses 0.0 0.0
Total, Fair Value 24.6 26.0
Total, Unrealized Losses 0.0 0.0
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value 125.3 148.8
Certificates of deposit and term deposits [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 97.8 66.4
Unrealized Gains 0.0 0.0
Unrealized Losses 0.0 0.0
Fair Value 97.8 66.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 17.3 13.0
Less Than 12 Months, Unrealized Losses (0.1) 0.0
12 Months or Greater, Fair Value 0.0 0.0
12 Months or Greater, Unrealized Losses 0.0 0.0
Total, Fair Value 17.3 13.0
Total, Unrealized Losses (0.1) 0.0
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value 97.8 66.4
U.S. government securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 166.3 195.0
Unrealized Gains 0.1 0.2
Unrealized Losses 0.0 0.0
Fair Value 166.4 195.2
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 42.1 47.1
Less Than 12 Months, Unrealized Losses 0.0 0.0
12 Months or Greater, Fair Value 0.0 0.0
12 Months or Greater, Unrealized Losses 0.0 0.0
Total, Fair Value 42.1 47.1
Total, Unrealized Losses 0.0 0.0
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value $ 166.4 $ 195.2