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Financial Instruments and Fair Value , Investments (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 951.2 $ 986.2
Unrealized Gains 1.0 1.3
Unrealized Losses (5.0) (0.1)
Fair Value 947.2 987.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 569.8 203.4
Less Than 12 Months, Unrealized Losses (5.0) (0.1)
12 Months or Greater, Fair Value 7.1 16.1
12 Months or Greater, Unrealized Losses 0.0 0.0
Total, Fair Value 576.9 219.5
Total, Unrealized Losses (5.0) (0.1)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Due within one year 775.2 843.1
Due within one to three years 172.0 144.3
Fair Value 947.2 987.4
Corporate debt securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 616.3 576.1
Unrealized Gains 0.3 1.0
Unrealized Losses (4.2) (0.1)
Fair Value 612.4 577.0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 475.2 117.3
Less Than 12 Months, Unrealized Losses (4.2) (0.1)
12 Months or Greater, Fair Value 7.1 16.1
12 Months or Greater, Unrealized Losses 0.0 0.0
Total, Fair Value 482.3 133.4
Total, Unrealized Losses (4.2) (0.1)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value 612.4 577.0
Commercial paper [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 150.6 148.7
Unrealized Gains 0.1 0.1
Unrealized Losses (0.1) 0.0
Fair Value 150.6 148.8
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 45.5 26.0
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 45.5 26.0
Total, Unrealized Losses (0.1) 0.0
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value 150.6 148.8
U.S. government securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 96.6 195.0
Unrealized Gains 0.5 0.2
Unrealized Losses 0.0 0.0
Fair Value 97.1 195.2
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value   47.1
Less Than 12 Months, Unrealized Losses   0.0
12 Months or Greater, Fair Value   0.0
12 Months or Greater, Unrealized Losses   0.0
Total, Fair Value   47.1
Total, Unrealized Losses   0.0
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value 97.1 195.2
Certificates of deposit and term deposits [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 87.7 66.4
Unrealized Gains 0.1 0.0
Unrealized Losses (0.7) 0.0
Fair Value 87.1 66.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 49.1 13.0
Less Than 12 Months, Unrealized Losses (0.7) 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 49.1 13.0
Total, Unrealized Losses (0.7) 0.0
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value $ 87.1 $ 66.4