INVESTMENT SECURITIES (Narrative) (Details)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2025 USD ($) security	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) security	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($) security
Marketable Securities [Line Items]
Accrued interest receivable	$ 57,000,000		$ 57,000,000		$ 60,000,000
Investment securities	19,438,000,000		19,438,000,000		$ 20,270,000,000
Unrealized loss amortization associated with the securities transferred from AFS to HTM	$ 47,000,000	$ 51,000,000	$ 137,000,000	$ 147,000,000
Number of AFS investment securities in an unrealized loss position \| security	2,135		2,135		2,534
Available-for-sale, realized gain (loss)	$ 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			1,700,000,000		$ 1,800,000,000
Unrealized loss amortization associated with the securities transferred from AFS to HTM			1,200,000,000		1,400,000,000
Asset Pledged as Collateral
Marketable Securities [Line Items]
Investment securities	$ 17,400,000,000		$ 17,400,000,000		$ 17,900,000,000

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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)
-Publisher FASB
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Amount, excluding accrued interest, of allowance for credit loss on investment in debt security measured at amortized cost (held-to-maturity).

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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