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Financial Instruments - Proceeds from Marketable Debt Securities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Investments, All Other Investments [Abstract]      
Proceeds from maturities and sales $ 7,299.4 $ 6,007.0 $ 9,173.7
Realized gains 17.7 6.0 3.2
Realized losses $ 26.0 $ 1.5 $ 11.7