BORROWINGS - Schedule of Long-term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Capital lease obligations	$ 15	$ 8
Total long-term borrowings	36,684	37,169
Loss (gain) on extinguishment of debt	2	0	$ (7)
Secured Debt
Debt Instrument [Line Items]
Loss (gain) on extinguishment of debt	$ 2
Parent | Fixed-to-Floating subordinated notes at 3.375% | Subordinated Debt
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.375%
Long-term debt, gross	$ 350	349
Parent | Junior subordinated debentures (FCB/SC Capital Trust II) | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	18
Subsidiaries | Senior unsecured fixed-to-floating rate notes at 2.969% | Senior Notes
Debt Instrument [Line Items]
Debt instrument, stated interest rate	2.969%
Long-term debt, gross	$ 0	318
Callable feature term	1 year
Subsidiaries | Fixed senior unsecured notes at 6.00% | Senior Notes
Debt Instrument [Line Items]
Debt instrument, stated interest rate	6.00%
Long-term debt, gross	$ 58	59
Subsidiaries | Fixed subordinated notes at 6.125% | Subordinated Debt
Debt Instrument [Line Items]
Debt instrument, stated interest rate	6.125%
Long-term debt, gross	$ 445	460
Subsidiaries | Fixed-to-Fixed subordinated notes at 4.125% | Subordinated Debt
Debt Instrument [Line Items]
Debt instrument, stated interest rate	4.125%
Long-term debt, gross	$ 0	101
Callable feature term	5 years
Subsidiaries | Junior subordinated debentures (SCB Capital Trust I) | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	10
Subsidiaries | Purchase Money Note to FDIC fixed at 3.50% | Secured Debt
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.50%
Long-term debt, gross	$ 35,816	$ 35,846