Derivative instruments - Gains (Losses) Related to Hedging Derivative Portfolio Recognized in the Income Statement (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Jun. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Derivative Instruments, Gain (Loss) [Line Items]
(Loss) gain recognized in the consolidated income statement due to fair value and cash flow hedging relationships	$ (3)	$ (1)	$ (1)	$ (7)	$ 5
Interest rate contracts \| Securities – Available-for-sale \| Interest income
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	(40)	(249)	(973)	(687)	(445)
Gain or (loss) recognized on hedged item	41	249	971	688	445
Interest rate contracts \| Long-term debt \| Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	98	275	643	784	435
Gain or (loss) recognized on hedged item	(98)	(275)	(643)	(784)	(435)
Interest rate contracts \| Loans \| Interest income
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	(2)	2	(25)	(11)	(40)
Gain or (loss) recognized on hedged item	2	(2)	25	11	40
FX contracts \| Staff expense
Derivative Instruments, Gain (Loss) [Line Items]
Loss (gain) on cash flow hedges	(1)	0	1	(5)	5
FX contracts \| Investment and other revenue
Derivative Instruments, Gain (Loss) [Line Items]
Loss (gain) on cash flow hedges	$ (3)	$ (1)	$ 0	$ (3)	$ 0

Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

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Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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Amount, before tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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