Accumulated Other Comprehensive Income (Loss) (Reclassification Adjustment Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment Security Gains (Losses), net	$ 0.3	$ 0.0	$ 0.5	$ 0.2
Other Operating Income	(38.9)	(31.4)	(67.9)	(66.2)
Interest Income	(640.2)	(567.7)	(1,303.0)	(1,073.6)
Employee Benefits	89.3	88.8	175.0	180.5
Net Income	(389.4)	(390.4)	(736.5)	(772.0)
Net Unrealized Gains (Losses) on Debt Securities Available for Sale | Net Gain/(Loss) Reclassified from AOCI to Net Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment Security Gains (Losses), net	0.1	0.0	0.3	0.0
Net Unrealized Gains (Losses) on Cash Flow Hedges | Net Gain/(Loss) Reclassified from AOCI to Net Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income	(3.8)	17.0	(17.4)	33.6
Amortization of Net Actuarial Loss | Net Gain/(Loss) Reclassified from AOCI to Net Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Employee Benefits	5.6	9.5	11.1	18.7
Amortization of Prior Service Cost | Net Gain/(Loss) Reclassified from AOCI to Net Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Employee Benefits	0.0	0.0	(0.1)	(0.1)
Net Pension and Other Postretirement Benefit Adjustments | Net Gain/(Loss) Reclassified from AOCI to Net Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income	5.6	9.5	11.0	18.6
Foreign Exchange Contracts | Net Unrealized Gains (Losses) on Cash Flow Hedges | Net Gain/(Loss) Reclassified from AOCI to Net Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Operating Income	(0.3)	(1.1)	0.3	(3.5)
Interest Income	(3.7)	(16.0)	(18.1)	(30.2)
Interest Rate Contracts | Net Unrealized Gains (Losses) on Cash Flow Hedges | Net Gain/(Loss) Reclassified from AOCI to Net Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest Income	$ 0.2	$ 0.1	$ 0.4	$ 0.1