Derivative Instruments and Hedging Activities - Gains (Losses) on Accounting Hedges and Fair Value Hedges (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Cash flow hedges - Interest rate contracts
Cash flow hedges expected to be reclassified from AOCI	$ (34.0)
Maximum length of time over which forecasted cash flows are hedged	37 months
Borrowings
Amortized cost	$ 576.0		$ 589.0
Hedged asset, unallocated basis adjustment	703.0		703.0
Amortized cost, unalloccated basis adjustment	0.3		2.0
Interest rate
Cash flow hedges - Interest rate contracts
Recognized in OCI	(338.0)	$ 17.0
Less: Realized gains (losses) (pre-tax) reclassified from AOCI to interest income	(4.0)	(5.0)
Net change in cash flow hedges included within AOCI	(334.0)	22.0
Investment securities—AFS
Fair Value Hedges - Hedged Items
Amortized cost basis/ carrying amount currently or previously hedged	50,016.0		55,451.0
Basis adjustments included in amortized cost/ carrying amount	37.0		217.0
Deposits
Fair Value Hedges - Hedged Items
Amortized cost basis/ carrying amount currently or previously hedged	59,109.0		53,224.0
Basis adjustments included in amortized cost/ carrying amount	(103.0)		149.0
Borrowings
Borrowings
Carrying amount currently or previously hedged	215,796.0		199,274.0
Basis adjustments included in carrying amount—Outstanding hedges	(7,257.0)		(6,252.0)
Basis adjustments included in carrying amount—Terminated hedges	(619.0)		$ (625.0)
Equity
Cash flow hedges - Interest rate contracts
Recognized in OCI	(58.0)	0.0
Less: Realized gains (losses) (pre-tax) reclassified from AOCI to interest income	(1.0)	0.0
Net change in cash flow hedges included within AOCI	(57.0)	0.0
Fair Value Hedges \| Interest Income \| Interest rate
Gain (Loss) on Fair Value Hedges Recognized in Interest Expense
Gains (losses) recognized in interest expense	292.0	(493.0)
Fair Value Hedges \| Interest Income \| Investment securities—AFS
Gain (Loss) on Fair Value Hedges Recognized in Interest Expense
Gains (losses) recognized in interest expense	(283.0)	503.0
Fair Value Hedges \| Interest Expense \| Interest rate
Gain (Loss) on Fair Value Hedges Recognized in Interest Expense
Gains (losses) recognized in interest expense	(1,253.0)	2,317.0
Fair Value Hedges \| Interest Expense \| Deposits
Gain (Loss) on Fair Value Hedges Recognized in Interest Expense
Gains (losses) recognized in interest expense	252.0	(49.0)
Fair Value Hedges \| Interest Expense \| Borrowings
Gain (Loss) on Fair Value Hedges Recognized in Interest Expense
Gains (losses) recognized in interest expense	1,009.0	(2,272.0)
Net Investment Hedges \| Foreign exchange
Net investment hedges—Foreign exchange contracts
Recognized in OCI	217.0	(435.0)
Forward points excluded from hedge effectiveness testing—Recognized in Interest income	$ 63.0	$ 17.0

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Amortized cost of beneficial interest secured by financial instrument, used to designate hedging relationship in which hedged item is expected to be outstanding for designated hedge period.

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The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

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Total amount of gain (loss) derived from fair value hedges recognized in earnings in the period.

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