GOODWILL AND INTANGIBLE ASSETS - Components of Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	$ 10,267	$ 10,371
Accumulated amortization of Intangible assets (excluding MSRs)	5,959	5,960
Net carrying amount of Intangible assets (excluding MSRs)	4,308	4,411
Gross carrying amount, Mortgage servicing rights (MSRs)	433	336
Mortgage servicing rights (MSRs)	433	336
Gross carrying amount of Intangible assets	10,700	10,707
Accumulated amortization of Intangible assets	5,959	5,960
Total intangible assets	4,741	4,747
Indefinite-lived intangible assets
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	185	190
Net carrying amount of Intangible assets (excluding MSRs)	185	190
Purchased credit card relationships
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	5,614	5,648
Accumulated amortization of Intangible assets (excluding MSRs)	4,239	4,229
Net carrying amount of Intangible assets (excluding MSRs)	1,375	1,419
Credit card contract related intangibles
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	3,906	3,929
Accumulated amortization of Intangible assets (excluding MSRs)	1,288	1,276
Net carrying amount of Intangible assets (excluding MSRs)	2,618	2,653
Core deposit intangibles
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	44	45
Accumulated amortization of Intangible assets (excluding MSRs)	44	44
Net carrying amount of Intangible assets (excluding MSRs)	0	1
Other customer relationships
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	426	455
Accumulated amortization of Intangible assets (excluding MSRs)	299	314
Net carrying amount of Intangible assets (excluding MSRs)	127	141
Present value of future profits
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	31	32
Accumulated amortization of Intangible assets (excluding MSRs)	29	30
Net carrying amount of Intangible assets (excluding MSRs)	2	2
Other
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	61	72
Accumulated amortization of Intangible assets (excluding MSRs)	60	67
Net carrying amount of Intangible assets (excluding MSRs)	$ 1	$ 5
Customer Concentration Risk | American Airlines, Costco, The Home Depot and AT&T | Intangible Assets, Excluding Mortgage Servicing Rights | Credit card contract related intangibles
Finite and Indefinite-lived Intangible Assets
Concentration risk, percentage	97.00%	96.00%