BORROWED FUNDS - Long Term Debt (Details) $ in Millions	6 Months Ended
	Jun. 30, 2021 USD ($) note	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Long-term borrowed funds	$ 6,957	$ 8,346
Private Exchange Offers
Debt Instrument [Line Items]
Principal amount	$ 265
Number of outstanding subordinated notes exchanged | note	5
Parent Company
Debt Instrument [Line Items]
Long-term borrowed funds	$ 3,095
Parent Company | Subordinated Debt | 4.150% fixed-rate subordinated debt, due September 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 168	182
Interest rate	4.15%
Parent Company | Subordinated Debt | 3.750% fixed-rate subordinated debt, due July 2024
Debt Instrument [Line Items]
Long-term borrowed funds	$ 90	159
Interest rate	3.75%
Parent Company | Subordinated Debt | 4.023% fixed-rate subordinated debt, due October 2024
Debt Instrument [Line Items]
Long-term borrowed funds	$ 17	25
Interest rate	4.023%
Parent Company | Subordinated Debt | 4.350% fixed-rate subordinated debt, due August 2025
Debt Instrument [Line Items]
Long-term borrowed funds	$ 133	193
Interest rate	4.35%
Parent Company | Subordinated Debt | 4.300% fixed-rate subordinated debt, due December 2025
Debt Instrument [Line Items]
Long-term borrowed funds	$ 336	450
Interest rate	4.30%
Parent Company | Subordinated Debt | 3.750% Fixed Rate Subordinated Debt, Due February 2031
Debt Instrument [Line Items]
Long-term borrowed funds	$ 69	0
Interest rate	3.75%
Parent Company | Subordinated Debt | 4.300% Fixed Rate Subordinated Debt, Due February 2031
Debt Instrument [Line Items]
Long-term borrowed funds	$ 135	0
Interest rate	4.30%
Parent Company | Subordinated Debt | 4.350% Fixed Rate Subordinated Debt, Due February 2031
Debt Instrument [Line Items]
Long-term borrowed funds	$ 60	0
Interest rate	4.35%
Parent Company | Subordinated Debt | 2.638% fixed-rate subordinated debt, due September 2032
Debt Instrument [Line Items]
Long-term borrowed funds	$ 547	543
Parent Company | Senior Unsecured Notes | 2.375% fixed-rate senior unsecured debt, due July 2021 (1)
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	350
Interest rate	2.375%
Parent Company | Senior Unsecured Notes | 2.850% fixed-rate senior unsecured notes, due July 2026
Debt Instrument [Line Items]
Long-term borrowed funds	$ 497	497
Interest rate	2.85%
Parent Company | Senior Unsecured Notes | 2.500% fixed-rate senior unsecured notes, due February 2030
Debt Instrument [Line Items]
Long-term borrowed funds	$ 298	297
Interest rate	2.50%
Parent Company | Senior Unsecured Notes | 3.250% fixed-rate senior unsecured notes, due April 2030
Debt Instrument [Line Items]
Long-term borrowed funds	$ 745	745
Interest rate	3.25%
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	$ 3,862
CBNA and Other Subsidiaries | Senior Unsecured Notes | 3.750% Fixed Rate Subordinated Debt, Due February 2031
Debt Instrument [Line Items]
Interest rate	3.75%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 4.300% Fixed Rate Subordinated Debt, Due February 2031
Debt Instrument [Line Items]
Interest rate	4.30%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 4.350% Fixed Rate Subordinated Debt, Due February 2031
Debt Instrument [Line Items]
Interest rate	4.35%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.550% senior unsecured notes, due May 2021
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	1,003
Interest rate	2.55%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 3.250% senior unsecured notes, due February 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 708	716
Interest rate	3.25%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 0.941% Floating Rate Senior Unsecured Notes, Due February 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 300	299
Interest rate	0.874%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 1.042% Floating Rate Senior Unsecured Notes, Due May 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 250	250
Interest rate	0.951%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.650% senior unsecured notes, due May 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 507	510
Interest rate	2.65%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 3.700% senior unsecured notes, due March 2023
Debt Instrument [Line Items]
Long-term borrowed funds	$ 520	527
Interest rate	3.70%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 1.201% Floating Rate Senior Unsecured Notes Due March 2023
Debt Instrument [Line Items]
Long-term borrowed funds	$ 250	249
Interest rate	1.096%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.250% senior unsecured notes, due April 2025
Debt Instrument [Line Items]
Long-term borrowed funds	$ 746	746
Interest rate	2.25%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 3.750% senior unsecured notes, due February 2026
Debt Instrument [Line Items]
Long-term borrowed funds	$ 536	551
Interest rate	3.75%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 3.750%, 4.300% And 4.350% Senior Notes Due 2031
Debt Instrument [Line Items]
Redeemable period prior to maturity	5 years
CBNA and Other Subsidiaries | Federal Home Loan Bank advances
Debt Instrument [Line Items]
Long-term borrowed funds	$ 18	19
Weighted average rate	0.909%
CBNA and Other Subsidiaries | Other
Debt Instrument [Line Items]
Long-term borrowed funds	$ 27	$ 35