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, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in the consolidated income statement due to fair value hedging relationships	$ 1	$ 0	$ 1	$ 0
Interest rate contracts \| Securities – Available-for-sale \| Income Statement Location [Axis]: us-gaap:InterestAndDividendIncomeOperating
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	222	(249)	440	(647)
Gain or (loss) recognized on hedged item	(221)	249	(439)	647
Interest rate contracts \| Long-term debt \| Income Statement Location [Axis]: us-gaap:InterestExpenseOperating
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	(212)	275	(354)	686
Gain or (loss) recognized on hedged item	212	(275)	354	(686)
Interest rate contracts \| Loans \| Income Statement Location [Axis]: us-gaap:InterestAndDividendIncomeOperating
Derivative Instruments, Gain (Loss) [Line Items]
Gain or (loss) recognized on derivatives	1	2	(4)	(9)
Gain or (loss) recognized on hedged item	$ (1)	$ (2)	$ 4	$ 9

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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