BORROWINGS - Schedule of Long-term Borrowings (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Debt Instrument [Line Items]
Capital lease obligations	$ 8		$ 9
Unamortized purchase accounting adjustments	(151)		(163)
Total long-term borrowings	37,145		37,169
Loss on extinguishment of debt	2	$ 0
Subordinated Debt
Debt Instrument [Line Items]
Loss 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		350
Parent | Junior subordinated debentures (FCB/SC Capital Trust II) | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0		20
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	$ 315		316
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	$ 51		51
Subsidiaries | Fixed subordinated notes at 6.125% | Subordinated Debt
Debt Instrument [Line Items]
Debt instrument, stated interest rate	6.125%
Long-term debt, gross	$ 400		404
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	$ 100		100
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	$ 36,072		$ 36,072