Accumulated Other Comprehensive Income (Loss) - Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment Security Losses, net	$ 1.0	$ 1.6	$ 3.2
Other Operating Income	(127.5)	(157.5)	(241.2)
Interest Income	(2,321.4)	(1,769.4)	(1,416.9)
Other Operating Expense	330.6	331.6	364.4
Net Income	(1,556.4)	(1,199.0)	(1,032.5)
Employee Benefits	356.7	319.9	293.3
Realized Losses (Gains) on Debt Securities Available for Sale | Gross Reclassification Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment Security Losses, net	0.5	1.4	(0.5)
Total Realized (Gains) on Cash Flow Hedges | Gross Reclassification Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income	(71.1)	(24.5)	(1.9)
Amortization of Net Actuarial Losses | Gross Reclassification Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Employee Benefits	36.6	26.0	25.6
Amortization of Prior Service Cost | Gross Reclassification Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Employee Benefits	(0.3)	(0.1)	(0.2)
Gross Reclassification Adjustment | Gross Reclassification Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income	36.3	25.9	25.4
Foreign Exchange Contracts | Total Realized (Gains) on Cash Flow Hedges | Gross Reclassification Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Operating Income	(3.9)	(5.0)	6.4
Interest Income	(67.4)	(19.3)	(6.4)
Other Operating Expense	0.0	0.1	0.9
Interest Rate Contracts | Total Realized (Gains) on Cash Flow Hedges | Gross Reclassification Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest Income	$ 0.2	$ (0.3)	$ (2.8)