Derivative instruments - Gains (Losses) Related to Hedging Derivative Portfolio Recognized in the Income Statement (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in the consolidated income statement due to fair value and cash flow hedging relationships	$ 7	$ 11	$ (2)
Interest rate contracts \| Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) on cash flow hedges	7	0	0
Interest rate contracts \| Long-term debt \| Interest expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	486	(328)	(197)
Gain or (loss) recognized on hedged item	(483)	330	190
Interest rate contracts \| Securities available-for-sale \| Interest revenue
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	(795)	284	82
Gain or (loss) recognized on hedged item	788	(273)	(97)
FX contracts \| Other revenue
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) on cash flow hedges	0	2	8
FX contracts \| Trading revenue
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) on cash flow hedges	0	0	2
FX contracts \| Staff expense
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) on cash flow hedges	3	(4)	10
FX contracts \| Securities available-for-sale
Derivative Instruments, Gain (Loss) [Line Items]
Gain from components excluded from assessment of fair value hedge effectiveness, net	2	(1)
FX contracts \| Securities available-for-sale \| Other revenue
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	9	(2)	0
Gain or (loss) recognized on hedged item	$ (8)	$ 2	$ 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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