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Derivative financial instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024	Dec. 31, 2023
Derivative [Line Items]
Notional Amount	$ 31,100		$ 31,100			$ 36,184
Aggregate fair value of derivative financial instruments in asset position	57		57			157
Collateral relating to net asset positions	58		58			157
Cash Flow Hedges
Derivative [Line Items]
Equity, attributable to parent, before tax	91	$ 59	91	$ 59	$ 85	(135)	$ (330)	$ (203)
Clearinghouse Credit Facilities
Derivative [Line Items]
Amount of initial margin posted	237		237			257
Interest Income
Derivative [Line Items]
Change in cash flow hedges derivative instruments, fair value net	(22)	(102)	(108)	(288)
Interest Rate Contracts | Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	44,800		44,800			40,500
Foreign exchange and other option and futures contracts | Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	2,000		2,000			$ 1,600
Interest rate swap agreements
Derivative [Line Items]
Cash flow hedge reclassification before tax	(33)	$ (115)	(139)	$ (328)
Interest rate swap agreements | Maturity Period 2026
Derivative [Line Items]
Equity, attributable to parent, before tax	(6)		(6)
Interest rate swap agreements | Maturity Period 2027
Derivative [Line Items]
Equity, attributable to parent, before tax	56		56
Interest rate swap agreements | Maturity Period 2028
Derivative [Line Items]
Equity, attributable to parent, before tax	$ 29		$ 29

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The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

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Fair value of securities received as collateral against derivative assets.

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