Derivative Financial Instruments - Notional and Fair Value Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Derivative [Line Items]
NOTIONAL VALUE	$ 340,241.0	$ 346,752.1
ASSET	1,902.7	4,097.2
Asset, Total Gross Derivative	2,347.4	4,558.2
Less: Asset, Netting	1,533.8	3,507.8
Derivative Assets	813.6	1,050.4
LIABILITY	1,566.5	3,738.0
Derivative Liabilities	2,135.2	4,882.8
Less: Liability, Netting	1,283.5	2,817.1
Derivative Liabilities	851.7	2,065.7
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	12,290.2	14,802.3
ASSET	252.7	23.8
LIABILITY	33.6	321.9
Derivatives Designated as Hedging under GAAP | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,430.5	4,717.6
ASSET	7.6	8.2
LIABILITY	8.7	10.2
Derivatives Designated as Hedging under GAAP | Cash Flow Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	3,697.9	6,554.4
ASSET	61.8	15.4
LIABILITY	12.6	104.0
Derivatives Designated as Hedging under GAAP | Cash Flow Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	0.0	50.0
ASSET	0.0	0.1
LIABILITY	0.0	0.0
Derivatives Designated as Hedging under GAAP | Net Investment Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,161.8	3,480.3
ASSET	183.3	0.1
LIABILITY	12.3	207.7
Total Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	327,950.8	331,949.8
ASSET	2,094.7	4,534.4
LIABILITY	2,101.6	4,560.9
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	848.4	813.1
ASSET	0.2	0.1
LIABILITY	38.1	35.4
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	109.9	67.7
ASSET	0.2	0.1
LIABILITY	0.6	0.1
Derivatives Not Designated as Hedging under GAAP | Fair Value Hedges | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	738.5	745.4
ASSET	0.0	0.0
LIABILITY	37.5	35.3
Client-Related and Trading Derivatives | Fair Value Hedges
Derivative [Line Items]
NOTIONAL VALUE	327,102.4	331,136.7
ASSET	2,094.5	4,534.3
LIABILITY	2,063.5	4,525.5
Client-Related and Trading Derivatives | Fair Value Hedges | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	315,532.3	320,563.4
ASSET	1,962.1	4,245.1
LIABILITY	1,973.3	4,410.7
Client-Related and Trading Derivatives | Fair Value Hedges | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	11,570.1	10,573.3
ASSET	132.4	289.2
LIABILITY	$ 90.2	$ 114.8