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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

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