BORROWED FUNDS - Long Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Long-term borrowed funds	$ 5,894	$ 6,932
Parent Company
Debt Instrument [Line Items]
Long-term borrowed funds	$ 3,101
Parent Company | Subordinated Debt | 4.150% fixed-rate subordinated debt, due September 2022
Debt Instrument [Line Items]
Interest rate	4.15%
Long-term borrowed funds	$ 168	168
Parent Company | Subordinated Debt | 3.750% fixed-rate subordinated debt, due July 2024
Debt Instrument [Line Items]
Interest rate	3.75%
Long-term borrowed funds	$ 90	90
Parent Company | Subordinated Debt | 4.023% fixed-rate subordinated debt, due October 2024
Debt Instrument [Line Items]
Interest rate	4.023%
Long-term borrowed funds	$ 17	17
Parent Company | Subordinated Debt | 4.350% fixed-rate subordinated debt, due August 2025
Debt Instrument [Line Items]
Interest rate	4.35%
Long-term borrowed funds	$ 133	133
Parent Company | Subordinated Debt | 4.300% fixed-rate subordinated debt, due December 2025
Debt Instrument [Line Items]
Interest rate	4.30%
Long-term borrowed funds	$ 337	336
Parent Company | Subordinated Debt | 3.750% fixed-rate reset subordinated debt, due February 2031
Debt Instrument [Line Items]
Interest rate	3.75%
Long-term borrowed funds	$ 69	69
Parent Company | Subordinated Debt | 4.300% fixed-rate reset subordinated debt, due February 2031
Debt Instrument [Line Items]
Interest rate	4.30%
Long-term borrowed funds	$ 135	135
Parent Company | Subordinated Debt | 4.350% fixed-rate reset subordinated debt, due February 2031
Debt Instrument [Line Items]
Interest rate	4.35%
Long-term borrowed funds	$ 60	60
Parent Company | Subordinated Debt | 2.638% fixed-rate subordinated debt, due September 2032
Debt Instrument [Line Items]
Long-term borrowed funds	$ 551	550
Parent Company | Senior Unsecured Notes | 2.850% fixed-rate senior unsecured notes, due July 2026
Debt Instrument [Line Items]
Interest rate	2.85%
Long-term borrowed funds	$ 498	498
Parent Company | Senior Unsecured Notes | 2.500% fixed-rate senior unsecured notes, due February 2030
Debt Instrument [Line Items]
Interest rate	2.50%
Long-term borrowed funds	$ 298	298
Parent Company | Senior Unsecured Notes | 3.250% fixed-rate senior unsecured notes, due April 2030
Debt Instrument [Line Items]
Interest rate	3.25%
Long-term borrowed funds	$ 745	745
Parent Company | Senior Unsecured Notes | 2.638% fixed-rate subordinated debt, due September 2032
Debt Instrument [Line Items]
Interest rate	2.638%
CBNA and Other Subsidiaries
Debt Instrument [Line Items]
Long-term borrowed funds	$ 2,793
CBNA and Other Subsidiaries | Senior Unsecured Notes | 3.250% senior unsecured notes, due February 2022
Debt Instrument [Line Items]
Interest rate	3.25%
Long-term borrowed funds	$ 0	700
CBNA and Other Subsidiaries | Senior Unsecured Notes | 0.845% floating-rate senior unsecured notes, due February 2022
Debt Instrument [Line Items]
Interest rate	0.845%
Long-term borrowed funds	$ 0	300
CBNA and Other Subsidiaries | Senior Unsecured Notes | 1.318% floating-rate senior unsecured notes, due May 2022
Debt Instrument [Line Items]
Interest rate	1.318%
Long-term borrowed funds	$ 250	250
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.650% senior unsecured notes, due May 2022
Debt Instrument [Line Items]
Interest rate	2.65%
Long-term borrowed funds	$ 500	503
CBNA and Other Subsidiaries | Senior Unsecured Notes | 3.700% senior unsecured notes, due March 2023
Debt Instrument [Line Items]
Interest rate	3.70%
Long-term borrowed funds	$ 504	512
CBNA and Other Subsidiaries | Senior Unsecured Notes | 1.933% floating-rate senior unsecured notes, due March 2023
Debt Instrument [Line Items]
Interest rate	1.933%
Long-term borrowed funds	$ 250	250
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.250% senior unsecured notes, due April 2025
Debt Instrument [Line Items]
Interest rate	2.25%
Long-term borrowed funds	$ 747	746
CBNA and Other Subsidiaries | Senior Unsecured Notes | 3.750% senior unsecured notes, due February 2026
Debt Instrument [Line Items]
Interest rate	3.75%
Long-term borrowed funds	$ 498	524
CBNA and Other Subsidiaries | Federal Home Loan Bank advances
Debt Instrument [Line Items]
Weighted average rate	0.774%
Long-term borrowed funds	$ 20	19
CBNA and Other Subsidiaries | Other
Debt Instrument [Line Items]
Long-term borrowed funds	$ 24	$ 29