BORROWED FUNDS - Long Term Debt (Details) $ in Millions	1 Months Ended
	Sep. 30, 2020 USD ($) note	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Long-term borrowed funds	$ 9,109	$ 14,047
Private Exchange Offers
Debt Instrument [Line Items]
Principal amount	$ 621
Number of outstanding subordinated notes exchanged | note	5
Payment for tender offers	$ 11
Parent Company
Debt Instrument [Line Items]
Long-term borrowed funds	3,439
Parent Company | Subordinated Debt | 4.150% fixed-rate subordinated debt, due September 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 181	348
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	$ 159	250
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	$ 25	42
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	$ 193	249
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	$ 450	750
Interest rate	4.30%
Parent Company | Subordinated Debt | 2.638% fixed-rate subordinated debt, due September 2032
Debt Instrument [Line Items]
Long-term borrowed funds	$ 542	0
Interest rate	2.638%
Parent Company | Senior Unsecured Notes | 2.375% fixed-rate senior unsecured debt, due July 2021
Debt Instrument [Line Items]
Long-term borrowed funds	$ 350	349
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	496
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	$ 297	0
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	0
Interest rate	3.25%
CBNA and Other Subsidiaries
Debt Instrument [Line Items]
Long-term borrowed funds	$ 5,670
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.250% senior unsecured notes, due March 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	700
Interest rate	2.25%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.678% floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	300
Interest rate	2.678%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.217% floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	250
Interest rate	2.217%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.200% senior unsecured notes, due May 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	500
Interest rate	2.20%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.250% senior unsecured notes, due October 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 750	750
Interest rate	2.25%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.550% senior unsecured notes, due May 2021
Debt Instrument [Line Items]
Long-term borrowed funds	$ 1,005	991
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	$ 720	711
Interest rate	3.25%
CBNA and Other Subsidiaries | Senior Unsecured Notes | .985% floating-rate senior unsecured notes, due February 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 299	299
Interest rate	0.985%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 1.044% floating-rate senior unsecured notes, due May 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 250	250
Interest rate	1.044%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.650% senior unsecured notes, due May 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 512	501
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	$ 530	515
Interest rate	3.70%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 1.168% floating-rate senior unsecured notes, due March 2023
Debt Instrument [Line Items]
Long-term borrowed funds	$ 249	249
Interest rate	1.168%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.250% senior unsecured notes, due April 2025
Debt Instrument [Line Items]
Long-term borrowed funds	$ 745	0
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	$ 555	521
Interest rate	3.75%
CBNA and Other Subsidiaries | Federal Home Loan Bank advances
Debt Instrument [Line Items]
Long-term borrowed funds	$ 19	5,008
Weighted average rate	0.943%
CBNA and Other Subsidiaries | Other
Debt Instrument [Line Items]
Long-term borrowed funds	$ 36	$ 18