Derivative instruments - Gains (Losses) Related to Hedging Derivative Portfolio Recognized in the Income Statement (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
(Loss) recognized in the consolidated income statement due to fair value and cash flow hedging relationships	$ (1)	$ (15)	$ (4)	$ (16)	$ (10)
Interest rate contracts \| Long-term debt \| Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	47	714	300	761	485
Gain or (loss) recognized on hedged item	(49)	(708)	(298)	(757)	(482)
Interest rate contracts \| Securities available-for-sale \| Interest revenue
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	19	(1,033)	(486)	(1,014)	(869)
Gain or (loss) recognized on hedged item	(15)	1,011	480	996	856
FX contracts \| Staff expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) on cash flow hedges	(3)	1	0	(2)	(1)
FX contracts \| Securities available-for-sale
Derivative Instruments, Gain (Loss) [Line Items]
Gain from components excluded from assessment of fair value hedge effectiveness, net	1	1		1	1
FX contracts \| Securities available-for-sale \| Other revenue
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	5	7	(5)	12	1
Gain or (loss) recognized on hedged item	$ (5)	$ (7)	$ 5	$ (12)	$ 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 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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