INVESTMENT SECURITIES AND OTHER SECURITIES - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)	Sep. 30, 2025 USD ($) security
Investment Securities [Line Items]
Number of municipal bonds classified as AFS debt securities \| security			1
AFS debt security charge-off		$ 2
Allowance related to investment securities, available for sale securities		0	$ 0
Recovery of previously written-off available-for-sale municipal security	$ 3
Allowance related to investment securities		0	0
Asset Pledged as Collateral
Investment Securities [Line Items]
Investment securities pledged		$ 37,700	$ 30,800

[Line Items] for Investment Securities [Table]

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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