BORROWED FUNDS - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Principal balance	$ 13,900	$ 13,600
Unamortized deferred issuance costs and/or discounts	75	74
Hedging basis adjustments	(19)	(17)
Short-term borrowed funds	9	505
Available borrowing capacity	70,600
Federal Home Loan Bank Advances and Letters of Credit | Secured Debt
Debt Instrument [Line Items]
Short-term borrowed funds	6,000	9,200
Federal Home Loan Bank advances
Debt Instrument [Line Items]
Available borrowing capacity	$ 18,700	$ 15,900