Derivative Financial Instruments - Notional Amounts and Fair Values for All Derivative Instruments Included in the Condensed Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Derivatives, Fair Value
Derivative Assets	$ 2,517	$ 2,919
Derivative Liabilities	1,227	1,072
Amount of variation margin payment applied to derivative asset contracts	962	1,100
Amount of variation margin payment applied to derivative liability contracts	778	1,100
Designated as Hedging Instrument | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Derivatives, Fair Value
Derivative Assets	578	788
Derivative Liabilities	2	2
Designated as Hedging Instrument | Fair Value Hedging | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Derivatives, Fair Value
Derivative Assets	383	528
Derivative Liabilities	0	0
Designated as Hedging Instrument | Fair Value Hedging | Interest rate swap | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Long-term debt
Derivatives, Fair Value
Notional Amount	1,955	1,955
Derivative Assets	383	528
Derivative Liabilities	0	0
Designated as Hedging Instrument | Cash Flow Hedging | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Derivatives, Fair Value
Derivative Assets	195	260
Derivative Liabilities	2	2
Designated as Hedging Instrument | Cash Flow Hedging | Interest rate swap | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Commercial and industrial
Derivatives, Fair Value
Notional Amount	8,000	8,000
Derivative Assets	0	16
Derivative Liabilities	2	2
Designated as Hedging Instrument | Cash Flow Hedging | Interest rate floor | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Commercial and industrial
Derivatives, Fair Value
Notional Amount	3,000	3,000
Derivative Assets	195	244
Derivative Liabilities	0	0
Not Designated as Hedging Instrument
Derivatives, Fair Value
Derivative Assets	1,939	2,131
Derivative Liabilities	1,225	1,070
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes
Derivatives, Fair Value
Derivative Assets	207	206
Derivative Liabilities	210	222
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Interest rate contracts related to MSR portfolio
Derivatives, Fair Value
Notional Amount	6,310	6,910
Derivative Assets	140	202
Derivative Liabilities	0	1
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Forward contracts related to residential mortgage loans held for sale | Loans held for sale
Derivatives, Fair Value
Notional Amount	6,872	2,903
Derivative Assets	59	1
Derivative Liabilities	8	16
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Swap
Derivatives, Fair Value
Notional Amount	3,473	3,588
Derivative Assets	0	0
Derivative Liabilities	195	201
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Foreign exchange contracts
Derivatives, Fair Value
Notional Amount	185	204
Derivative Assets	0	0
Derivative Liabilities	1	3
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Interest rate contracts for collateral management
Derivatives, Fair Value
Notional Amount	12,000	12,000
Derivative Assets	8	3
Derivative Liabilities	6	1
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Commercial loan trading
Derivatives, Fair Value
Notional Amount	18
Derivative Assets	0
Derivative Liabilities	0
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Interest rate contracts for LIBOR transition
Derivatives, Fair Value
Notional Amount	2,372	2,372
Derivative Assets	0	0
Derivative Liabilities	0	0
Not Designated as Hedging Instrument | Customer Accommodation
Derivatives, Fair Value
Derivative Assets	1,732	1,925
Derivative Liabilities	1,015	848
Not Designated as Hedging Instrument | Customer Accommodation | Foreign exchange contracts
Derivatives, Fair Value
Notional Amount	16,455	14,587
Derivative Assets	250	255
Derivative Liabilities	214	224
Not Designated as Hedging Instrument | Customer Accommodation | Interest rate contracts
Derivatives, Fair Value
Notional Amount	79,055	77,806
Derivative Assets	881	1,238
Derivative Liabilities	256	265
Amount of variation margin payment applied to derivative asset contracts	83	47
Amount of variation margin payment applied to derivative liability contracts	736	1,063
Not Designated as Hedging Instrument | Customer Accommodation | Interest rate lock commitments
Derivatives, Fair Value
Notional Amount	1,959	1,830
Derivative Assets	39	57
Derivative Liabilities	1	0
Not Designated as Hedging Instrument | Customer Accommodation | Commodity contracts
Derivatives, Fair Value
Notional Amount	8,440	7,762
Derivative Assets	562	375
Derivative Liabilities	544	$ 359
Not Designated as Hedging Instrument | Customer Accommodation | TBA securities
Derivatives, Fair Value
Notional Amount	30
Derivative Assets	0
Derivative Liabilities	$ 0