INVESTMENT SECURITIES (Narrative) (Details)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2026 USD ($) security	Sep. 30, 2025 USD ($)	Sep. 30, 2026 USD ($) security	Sep. 30, 2025 USD ($)	Dec. 31, 2025 USD ($) security
Marketable Securities [Line Items]
Accrued interest receivable	$ 58,000,000		$ 58,000,000		$ 64,000,000
Investment securities	18,617,000,000		18,617,000,000		$ 19,220,000,000
Unrealized loss amortization associated with the securities transferred from AFS to HTM	$ 44,000,000	$ 47,000,000	$ 127,000,000	$ 137,000,000
Number of AFS investment securities in an unrealized loss position \| security	1,940		1,940		2,037
Available-for-sale, gross gains	$ 0	0	$ 0	0
Available-for-sale, gross losses	0	$ 0	0	$ 0
Allowance for credit loss on HTM securities (less than)	1,000,000		1,000,000
Fixed-Rate Available For Sale Securities
Marketable Securities [Line Items]
Unrealized loss amortization associated with the securities transferred from AFS to HTM, before tax			1,400,000,000		$ 1,600,000,000
Unrealized loss amortization associated with the securities transferred from AFS to HTM			1,100,000,000		1,200,000,000
Asset Pledged as Collateral
Marketable Securities [Line Items]
Investment securities	$ 16,500,000,000		$ 16,500,000,000		$ 17,500,000,000

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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