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, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in the consolidated income statement due to fair value and cash flow hedging relationships	$ 1	$ 2	$ 10	$ 13	$ (6)
Interest rate contracts \| Long-term debt \| Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	(96)	22	(68)	(427)	693
Gain or (loss) recognized on hedged item	96	(21)	66	426	(691)
Interest rate contracts \| Securities available-for-sale \| Interest revenue
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	183	(325)	150	649	(864)
Gain or (loss) recognized on hedged item	(184)	322	(140)	(647)	856
FX contracts \| Staff expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) on cash flow hedges	2	4	0	11	(2)
FX contracts \| Securities available-for-sale
Derivative Instruments, Gain (Loss) [Line Items]
Gain from components excluded from assessment of fair value hedge effectiveness, net (less than)	1	1	1	1	1
FX contracts \| Securities available-for-sale \| Foreign exchange revenue
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	0	(1)	1	7	(11)
Gain or (loss) recognized on hedged item	$ 0	$ 1	$ 1	$ (6)	$ 13

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 of increase (decrease) in the fair value of derivatives recognized in the income statement.

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Net gain (loss) relating to components of the gain (loss) on the fair value hedging instrument excluded from the assessment of fair value hedge effectiveness. Recognized in earnings.

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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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