BORROWED FUNDS - Long Term Debt (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) note	Dec. 31, 2021 USD ($) note
Debt Instrument [Line Items]
Long-term borrowed funds	$ 8,346	$ 6,932
Private Exchange Offers
Debt Instrument [Line Items]
Principal amount	$ 621	$ 265
Number of outstanding subordinated notes exchanged | note	5	5
Payment for tender offers	$ 11
Parent company
Debt Instrument [Line Items]
Long-term borrowed funds		$ 3,099
Parent company | Subordinated Debt | 4.150% fixed rate subordinated debt, due 2022
Debt Instrument [Line Items]
Long-term borrowed funds	182	$ 168
Interest rate		4.15%
Parent company | Subordinated Debt | 3.750% fixed rate subordinated debt due 2024
Debt Instrument [Line Items]
Long-term borrowed funds	159	$ 90
Interest rate		3.75%
Parent company | Subordinated Debt | 4.023% fixed rate subordinated debt, due 2024
Debt Instrument [Line Items]
Long-term borrowed funds	25	$ 17
Interest rate		4.023%
Parent company | Subordinated Debt | 4.350% fixed rate subordinated debt, due 2025
Debt Instrument [Line Items]
Long-term borrowed funds	193	$ 133
Interest rate		4.35%
Parent company | Subordinated Debt | 4.300% fixed rate subordinated debt, due 2025
Debt Instrument [Line Items]
Long-term borrowed funds	450	$ 336
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	$ 543	$ 550
Interest rate	2.638%
Parent company | Subordinated Debt | 3.750% Fixed Rate Subordinated Debt, Due February 2031
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	$ 69
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	0	$ 135
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	0	$ 60
Interest rate		4.35%
Parent company | Senior Unsecured Notes | 2.375% fixed rate senior unsecured debt, due 2021
Debt Instrument [Line Items]
Long-term borrowed funds	350	$ 0
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	$ 498
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	$ 298
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%
CBNA and Other Subsidiaries
Debt Instrument [Line Items]
Long-term borrowed funds		$ 3,833
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.550% senior unsecured notes, due May 2021
Debt Instrument [Line Items]
Long-term borrowed funds	1,003	$ 0
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	716	$ 700
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	299	$ 300
Interest rate		0.845%
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		0.932%
CBNA and Other Subsidiaries | Senior Unsecured Notes | 2.650% senior unsecured notes, due May 2022
Debt Instrument [Line Items]
Long-term borrowed funds	510	$ 503
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	527	$ 512
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	249	$ 250
Interest rate		1.17%
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	551	$ 524
Interest rate		3.75%
CBNA and Other Subsidiaries | Federal Home Loan advances
Debt Instrument [Line Items]
Long-term borrowed funds	19	$ 19
Weighted average interest rate		0.852%
CBNA and Other Subsidiaries | Other
Debt Instrument [Line Items]
Long-term borrowed funds	$ 35	$ 29