Borrowings - Schedule of Long-term Borrowings (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Capital lease obligations	$ 7	$ 7
Unamortized issuance costs	(1)	(1)
Unamortized purchase accounting adjustment	139	(87)
Total long-term borrowings	45,085	4,459
Subordinated Debt
Debt Instrument [Line Items]
Unamortized purchase accounting adjustment	65	69
Parent | Fixed-to-Floating subordinated notes at 3.375% | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 350	350
Debt instrument, stated interest rate	3.375%
Parent | Junior subordinated debentures at 3-month LIBOR plus 2.25% (FCB/SC Capital Trust II) | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 20	20
Parent | Junior subordinated debentures at 3-month LIBOR plus 2.25% (FCB/SC Capital Trust II) | Subordinated Debt | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
Parent | Junior subordinated debentures at 3-month LIBOR plus 1.75% (FCB/NC Capital Trust III) | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 88	88
Parent | Junior subordinated debentures at 3-month LIBOR plus 1.75% (FCB/NC Capital Trust III) | Subordinated Debt | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
Subsidiaries | Senior unsecured fixed to floating rate notes at 3.929% | Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 500	500
Debt instrument, stated interest rate	3.929%
Subsidiaries | Senior unsecured fixed to floating rate notes at 2.969% | Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 315	315
Debt instrument, stated interest rate	2.969%
Subsidiaries | Fixed senior unsecured notes at 6.00% | Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 51	51
Debt instrument, stated interest rate	6.00%
Subsidiaries | Fixed subordinated notes at 6.125% | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 400	400
Debt instrument, stated interest rate	6.125%
Subsidiaries | Fixed-to-Fixed subordinated notes at 4.125% | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 100	100
Debt instrument, stated interest rate	4.125%
Subsidiaries | Junior subordinated debentures at 3-month LIBOR plus 2.80% (Macon Capital Trust I) | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 14	14
Subsidiaries | Junior subordinated debentures at 3-month LIBOR plus 2.80% (Macon Capital Trust I) | Subordinated Debt | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	2.80%
Subsidiaries | Junior subordinated debentures at 3-month LIBOR plus 2.85% (SCB Capital Trust I) | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 10	10
Subsidiaries | Junior subordinated debentures at 3-month LIBOR plus 2.85% (SCB Capital Trust I) | Subordinated Debt | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	2.85%
Subsidiaries | Notes payable to FHLB of Atlanta at overnight SOFR plus spreads ranging from 0.24% to 0.48%. | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 8,000	2,500
Subsidiaries | Notes payable to FHLB of Atlanta at overnight SOFR plus spreads ranging from 0.24% to 0.48%. | Secured Debt | Secured Overnight Financing Rate (SOFR) | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.24%
Subsidiaries | Notes payable to FHLB of Atlanta at overnight SOFR plus spreads ranging from 0.24% to 0.48%. | Secured Debt | Secured Overnight Financing Rate (SOFR) | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	0.48%
Subsidiaries | Fixed senior unsecured notes at 3.50% | Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross	$ 35,370	0
Debt instrument, stated interest rate	3.50%
Subsidiaries | Other secured financings | Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 18