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Financial Instruments and Fair Value , Investments (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 707.6 $ 986.2
Unrealized Gains 3.5 1.3
Unrealized Losses (0.2) (0.1)
Fair Value 710.9 987.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 82.9 203.4
Less Than 12 Months, Unrealized Losses (0.2) (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 82.9 219.5
Total, Unrealized Losses (0.2) (0.1)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Due within one year 610.5 843.1
Due within one to three years 100.4 144.3
Fair Value 710.9 987.4
Corporate debt securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 502.2 576.1
Unrealized Gains 3.0 1.0
Unrealized Losses (0.1) (0.1)
Fair Value 505.1 577.0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 62.6 117.3
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 62.6 133.4
Total, Unrealized Losses (0.1) (0.1)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value 505.1 577.0
Commercial paper [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 75.6 148.7
Unrealized Gains 0.2 0.1
Unrealized Losses 0.0 0.0
Fair Value 75.8 148.8
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 2.0 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 2.0 26.0
Total, Unrealized Losses 0.0 0.0
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value 75.8 148.8
Certificates of deposit and term deposits [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 69.0 66.4
Unrealized Gains 0.1 0.0
Unrealized Losses (0.1) 0.0
Fair Value 69.0 66.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Less Than 12 Months, Fair Value 18.3 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 18.3 13.0
Total, Unrealized Losses (0.1) 0.0
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Fair Value 69.0 66.4
U.S. government securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 60.8 195.0
Unrealized Gains 0.2 0.2
Unrealized Losses 0.0 0.0
Fair Value 61.0 195.2
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 $ 61.0 $ 195.2