DESCRIPTION OF BUSINESS, BASIS OF PRESENTATION AND ACCOUNTING POLICIES - Schedule of Finite-Lived Intangible Assets (Details) - SHUSA $ in Millions	9 Months Ended
Sep. 30, 2026 USD ($)
Business Combination [Line Items]
Amount	$ 2,264.0
Trade names
Business Combination [Line Items]
Amount	10.0
Core deposit intangibles - consumer and commercial
Business Combination [Line Items]
Amount	$ 1,441.0
Core deposit intangibles - consumer and commercial \| Minimum
Business Combination [Line Items]
Selected Useful Life	7 years
Core deposit intangibles - consumer and commercial \| Maximum
Business Combination [Line Items]
Selected Useful Life	11 years
Core deposit intangibles - health savings accounts
Business Combination [Line Items]
Amount	$ 517.0
Selected Useful Life	8 years
Core deposit intangibles - workers comp administration funds
Business Combination [Line Items]
Amount	$ 234.0
Selected Useful Life	16 years
Customer relationships
Business Combination [Line Items]
Amount	$ 72.0
Selected Useful Life	10 years

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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