Derivative Financial Instruments - Location and Amount of Fair Value Hedge Derivative Gains and Losses Recorded in Income (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Interest Income	$ 2,321.4	$ 1,643.5	$ 2,499.9	$ 2,321.4
Interest Expense	698.7	200.3	822.0	698.7
Other Operating Income	127.5	194.0	145.5	$ 127.5
INTEREST INCOME
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) recognized on fair value hedges	17.8	(13.2)	21.2
Total gain/(loss) recognized on cash flow hedges	67.2	27.9	25.9
INTEREST EXPENSE
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) recognized on fair value hedges	7.9	29.9	5.2
Total gain/(loss) recognized on cash flow hedges	0.0	0.0	0.0
OTHER OPERATING INCOME
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) recognized on fair value hedges	0.0	0.0	0.0
Total gain/(loss) recognized on cash flow hedges	3.9	0.2	0.8
Interest Rate Contracts | INTEREST INCOME
Derivative Instruments, Gain (Loss) [Line Items]
Interest Rate Contracts, Recognized on Derivatives	13.9	(66.3)	(95.9)
Interest Rate Contracts, Recognized on Hedged Items	(13.9)	66.3	95.9
Interest Rate Contracts, Amounts Related to Interest Settlements on Derivatives	17.8	(13.2)	21.2
Total gain/(loss) recognized on cash flow hedges	(0.2)	0.5	(0.5)
Interest Rate Contracts | INTEREST EXPENSE
Derivative Instruments, Gain (Loss) [Line Items]
Interest Rate Contracts, Recognized on Derivatives	(9.5)	100.2	99.4
Interest Rate Contracts, Recognized on Hedged Items	9.5	(100.2)	(99.4)
Interest Rate Contracts, Amounts Related to Interest Settlements on Derivatives	7.9	29.9	5.2
Total gain/(loss) recognized on cash flow hedges	0.0	0.0	0.0
Interest Rate Contracts | OTHER OPERATING INCOME
Derivative Instruments, Gain (Loss) [Line Items]
Interest Rate Contracts, Recognized on Derivatives	0.0	0.0	0.0
Interest Rate Contracts, Recognized on Hedged Items	0.0	0.0	0.0
Interest Rate Contracts, Amounts Related to Interest Settlements on Derivatives	0.0	0.0	0.0
Total gain/(loss) recognized on cash flow hedges	0.0	0.0	0.0
Foreign Exchange Contracts | INTEREST INCOME
Derivative Instruments, Gain (Loss) [Line Items]
Total gain/(loss) recognized on cash flow hedges	67.4	27.4	26.4
Foreign Exchange Contracts | INTEREST EXPENSE
Derivative Instruments, Gain (Loss) [Line Items]
Total gain/(loss) recognized on cash flow hedges	0.0	0.0	0.0
Foreign Exchange Contracts | OTHER OPERATING INCOME
Derivative Instruments, Gain (Loss) [Line Items]
Total gain/(loss) recognized on cash flow hedges	$ 3.9	$ 0.2	$ 0.8