Derivative Financial Instruments - Notional Amounts and Fair Values for All Derivative Instruments Included in the Condensed Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivatives, Fair Value
Derivative Assets	$ 3,961	$ 2,919
Derivative Liabilities	2,738	1,072
Amount of variation margin payment applied to derivative asset contracts	862	1,100
Amount of variation margin payment applied to derivative liability contracts	710	1,100
Designated as Hedging Instrument | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Derivatives, Fair Value
Derivative Assets	580	788
Derivative Liabilities	1	2
Designated as Hedging Instrument | Fair Value Hedging | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Derivatives, Fair Value
Derivative Assets	414	528
Derivative Liabilities	0	0
Designated as Hedging Instrument | Fair Value Hedging | Interest rate swap | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Available-for-sale debt and other securities
Derivatives, Fair Value
Notional Amount	445
Derivative Assets	9
Derivative Liabilities	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	1,455	1,955
Derivative Assets	405	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	166	260
Derivative Liabilities	1	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	1	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	166	244
Derivative Liabilities	0	0
Not Designated as Hedging Instrument
Derivatives, Fair Value
Derivative Assets	3,381	2,131
Derivative Liabilities	2,737	1,070
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes
Derivatives, Fair Value
Derivative Assets	175	206
Derivative Liabilities	222	222
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Interest rate contracts related to MSR portfolio
Derivatives, Fair Value
Notional Amount	6,485	6,910
Derivative Assets	146	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	4,102	2,903
Derivative Assets	20	1
Derivative Liabilities	4	16
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Swap
Derivatives, Fair Value
Notional Amount	3,654	3,588
Derivative Assets	0	0
Derivative Liabilities	211	201
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Foreign exchange contracts
Derivatives, Fair Value
Notional Amount	154	204
Derivative Assets	1	0
Derivative Liabilities	0	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 | Interest rate contracts for LIBOR transition
Derivatives, Fair Value
Notional Amount	2,372	2,372
Derivative Assets	0	0
Derivative Liabilities	1	0
Not Designated as Hedging Instrument | Customer Accommodation
Derivatives, Fair Value
Derivative Assets	3,206	1,925
Derivative Liabilities	2,515	848
Not Designated as Hedging Instrument | Customer Accommodation | Foreign exchange contracts
Derivatives, Fair Value
Notional Amount	20,888	14,587
Derivative Assets	284	255
Derivative Liabilities	251	224
Not Designated as Hedging Instrument | Customer Accommodation | Interest rate contracts
Derivatives, Fair Value
Notional Amount	79,879	77,806
Derivative Assets	772	1,238
Derivative Liabilities	215	265
Amount of variation margin payment applied to derivative asset contracts	94	47
Amount of variation margin payment applied to derivative liability contracts	675	1,063
Not Designated as Hedging Instrument | Customer Accommodation | Interest rate lock commitments
Derivatives, Fair Value
Notional Amount	1,500	1,830
Derivative Assets	27	57
Derivative Liabilities	0	0
Not Designated as Hedging Instrument | Customer Accommodation | Commodity contracts
Derivatives, Fair Value
Notional Amount	11,933	7,762
Derivative Assets	2,123	375
Derivative Liabilities	2,049	$ 359
Not Designated as Hedging Instrument | Customer Accommodation | TBA securities
Derivatives, Fair Value
Notional Amount	24
Derivative Assets	0
Derivative Liabilities	$ 0