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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment Security Gains (Losses), net (Note)	$ 0.0	$ 0.4	$ 0.2	$ 0.7
Other Operating Income	(31.4)	(43.0)	(66.2)	(82.7)
Interest Income	(567.7)	(417.2)	(1,073.6)	(827.5)
Other Operating Expense	75.3	82.3	144.3	147.4
Employee Benefits	88.8	75.6	180.5	153.4
Net Income	(390.4)	(267.9)	(772.0)	(544.0)
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 (Note)	0.0	0.4	0.0	0.6
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	(17.0)	5.4	(33.6)	10.5
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	9.5	6.5	18.7	13.0
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	0.2	(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	9.5	6.7	18.6	13.1
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	(1.1)	(1.8)	(3.5)	(2.5)
Interest Income	(16.0)	(3.5)	(30.2)	(7.9)
Other Operating Expense	0.0	0.0	0.0	0.1
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	$ 0.1	$ (0.1)	$ 0.1	$ (0.2)