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Financial Instruments (Details 3) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Proceeds from Marketable Debt Securities |  |  |
Proceeds from maturities and sales | $ 5,565.9 | $ 7,378.9 | $ 4,063.0
Realized gains | 3.0 | 3.3 | 1.5
Realized losses | $ (22.4) | $ (4.3) | $ (3.5)

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