GOODWILL AND INTANGIBLE ASSETS - Components of Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	$ 9,795	$ 12,271
Accumulated amortization of Intangible assets (excluding MSRs)	5,602	7,944
Net carrying amount of Intangible assets (excluding MSRs)	4,193	4,327
Gross carrying amount, Mortgage servicing rights (MSRs)	367	495
Mortgage servicing rights (MSRs)	367	495
Gross carrying amount of Intangible assets	10,162	12,766
Accumulated amortization of Intangible assets	5,602	7,944
Total intangible assets	4,560	4,822
Indefinite-lived intangible assets
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	191	228
Accumulated amortization of Intangible assets (excluding MSRs)	0	0
Net carrying amount of Intangible assets (excluding MSRs)	191	228
Purchased credit card relationships
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	5,634	5,676
Accumulated amortization of Intangible assets (excluding MSRs)	4,060	4,059
Net carrying amount of Intangible assets (excluding MSRs)	1,574	1,617
Credit card contract related intangibles
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	3,417	5,393
Accumulated amortization of Intangible assets (excluding MSRs)	1,138	3,069
Net carrying amount of Intangible assets (excluding MSRs)	2,279	2,324
Core deposit intangibles
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	42	434
Accumulated amortization of Intangible assets (excluding MSRs)	42	433
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)	424	424
Accumulated amortization of Intangible assets (excluding MSRs)	281	275
Net carrying amount of Intangible assets (excluding MSRs)	143	149
Present value of future profits
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	27	34
Accumulated amortization of Intangible assets (excluding MSRs)	24	31
Net carrying amount of Intangible assets (excluding MSRs)	3	3
Other
Finite and Indefinite-lived Intangible Assets
Gross carrying amount of Intangible assets (excluding MSRs)	60	82
Accumulated amortization of Intangible assets (excluding MSRs)	57	77
Net carrying amount of Intangible assets (excluding MSRs)	$ 3	$ 5
Customer Concentration Risk | American Airlines, Sears, The Home Depot, Costco and AT&T | Intangible Assets, Excluding Mortgage Servicing Rights | Credit card contract related intangibles
Finite and Indefinite-lived Intangible Assets
Concentration risk, percentage	96.00%	96.00%