Derivative Financial Instruments (Carrying Value of Hedged Items and Associated Cumulative Hedging Adjustment Relating for Fair Value Hedges) (Parenthetical) (Details) - Fair Value Hedges - Carrying Value - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Long-term Debt	$ 1,752.1	$ 1,747.0
Derivatives not designated as hedging instruments (Non-qualifying Hedges)
Derivative Instruments, Gain (Loss) [Line Items]
Long-term Debt	$ 1,246.4	$ 499.4