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, 2025	Jul. 21, 2025	Dec. 31, 2024
Derivatives, Fair Value
Derivative Assets	$ 1,837		$ 2,472
Derivative Liabilities	2,055		2,798
Amount of variation margin payment applied to derivative asset contracts	302		403
Amount of variation margin payment applied to derivative liability contracts	381		1,200
Forward contracts related to residential mortgage loans measured at fair value
Derivatives, Fair Value
Notional Amount		$ 150
Designated as Hedging Instrument
Derivatives, Fair Value
Derivative Assets	1		7
Derivative Liabilities	13		16
Designated as Hedging Instrument | Fair Value Hedging
Derivatives, Fair Value
Derivative Assets	1		1
Derivative Liabilities	12		12
Designated as Hedging Instrument | Fair Value Hedging | Interest rate swap | Long-term debt
Derivatives, Fair Value
Notional Amount	4,205		4,955
Derivative Assets	1		1
Derivative Liabilities	12		12
Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value
Derivative Assets	0		6
Derivative Liabilities	1		4
Designated as Hedging Instrument | Cash Flow Hedging | Interest rate swap | Commercial and industrial
Derivatives, Fair Value
Notional Amount	6,850		11,000
Derivative Assets	0		2
Derivative Liabilities	1		4
Designated as Hedging Instrument | Cash Flow Hedging | Interest rate swap | Commercial mortgage and commercial construction
Derivatives, Fair Value
Notional Amount	4,000
Derivative Assets	0
Derivative Liabilities	0
Designated as Hedging Instrument | Cash Flow Hedging | Forward starting interest rate swap | Commercial and industrial
Derivatives, Fair Value
Notional Amount			1,000
Derivative Assets			1
Derivative Liabilities			0
Designated as Hedging Instrument | Cash Flow Hedging | Forward starting interest rate swap | Commercial mortgage and commercial construction
Derivatives, Fair Value
Notional Amount			4,000
Derivative Assets			3
Derivative Liabilities			0
Not Designated as Hedging Instrument
Derivatives, Fair Value
Derivative Assets	1,836		2,465
Derivative Liabilities	2,042		2,782
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes
Derivatives, Fair Value
Derivative Assets	13		15
Derivative Liabilities	157		174
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Interest rate contracts related to MSR portfolio
Derivatives, Fair Value
Notional Amount	4,750		3,135
Derivative Assets	10		4
Derivative Liabilities	2		4
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Forward contracts related to residential mortgage loans measured at fair value | Loans Measured At Fair Value
Derivatives, Fair Value
Notional Amount	1,412		881
Derivative Assets	2		8
Derivative Liabilities	5		0
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Swap
Derivatives, Fair Value
Notional Amount	2,627		2,465
Derivative Assets	0		0
Derivative Liabilities	150		170
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Foreign exchange contracts
Derivatives, Fair Value
Notional Amount	124		104
Derivative Assets	1		2
Derivative Liabilities	0		0
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Interest-only strips
Derivatives, Fair Value
Notional Amount	24		30
Derivative Assets	0		0
Derivative Liabilities	0		0
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Interest rate contracts for collateral management
Derivatives, Fair Value
Notional Amount			1,000
Derivative Assets			1
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	597		597
Derivative Assets	0		0
Derivative Liabilities	0		0
Not Designated as Hedging Instrument | Risk Management and Other Business Purposes | Other
Derivatives, Fair Value
Notional Amount	48		43
Derivative Assets	0		0
Derivative Liabilities	0		0
Not Designated as Hedging Instrument | Customer Accommodation
Derivatives, Fair Value
Derivative Assets	1,823		2,450
Derivative Liabilities	1,885		2,608
Not Designated as Hedging Instrument | Customer Accommodation | Foreign exchange contracts
Derivatives, Fair Value
Notional Amount	29,158		38,640
Derivative Assets	667		1,165
Derivative Liabilities	628		1,120
Not Designated as Hedging Instrument | Customer Accommodation | Interest rate contracts
Derivatives, Fair Value
Notional Amount	81,917		87,928
Derivative Assets	481		708
Derivative Liabilities	585		924
Amount of variation margin payment applied to derivative asset contracts	132		257
Amount of variation margin payment applied to derivative liability contracts	33		45
Not Designated as Hedging Instrument | Customer Accommodation | Interest rate lock commitments
Derivatives, Fair Value
Notional Amount	562		264
Derivative Assets	8		2
Derivative Liabilities	0		0
Not Designated as Hedging Instrument | Customer Accommodation | Commodity contracts
Derivatives, Fair Value
Notional Amount	17,379		16,889
Derivative Assets	667		575
Derivative Liabilities	672		564
Not Designated as Hedging Instrument | Customer Accommodation | TBA securities
Derivatives, Fair Value
Notional Amount	28		44
Derivative Assets	0		0
Derivative Liabilities	$ 0		$ 0