GOODWILL AND INTANGIBLE ASSETS - Changes in Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2018, Intangible assets (excluding MSRs)	$ 4,636
Acquisitions/ divestitures	0
Amortization	(144)
FX translation and other	153
Net carrying amount at March 31, 2019, Intangible assets (excluding MSRs)	4,645	$ 4,636
Mortgage servicing rights (MSRs)	551	584
Total intangible assets	5,196	5,220
Indefinite-lived intangible assets
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2018, Intangible assets (excluding MSRs)	218
Acquisitions/ divestitures	0
Amortization	0
FX translation and other	2
Net carrying amount at March 31, 2019, Intangible assets (excluding MSRs)	220	218
Purchased credit card relationships
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2018, Intangible assets (excluding MSRs)	1,797
Acquisitions/ divestitures	0
Amortization	(48)
FX translation and other	0
Net carrying amount at March 31, 2019, Intangible assets (excluding MSRs)	1,749	1,797
Credit card contract related intangibles
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2018, Intangible assets (excluding MSRs)	2,434
Acquisitions/ divestitures	0
Amortization	(84)
FX translation and other	150
Net carrying amount at March 31, 2019, Intangible assets (excluding MSRs)	2,500	2,434
Core deposit intangibles
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2018, Intangible assets (excluding MSRs)	4
Acquisitions/ divestitures	0
Amortization	(2)
FX translation and other	0
Net carrying amount at March 31, 2019, Intangible assets (excluding MSRs)	2	4
Other customer relationships
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2018, Intangible assets (excluding MSRs)	171
Acquisitions/ divestitures	0
Amortization	(6)
FX translation and other	0
Net carrying amount at March 31, 2019, Intangible assets (excluding MSRs)	165	171
Present value of future profits
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2018, Intangible assets (excluding MSRs)	3
Acquisitions/ divestitures	0
Amortization	0
FX translation and other	1
Net carrying amount at March 31, 2019, Intangible assets (excluding MSRs)	4	3
Other
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2018, Intangible assets (excluding MSRs)	9
Acquisitions/ divestitures	0
Amortization	(4)
FX translation and other	0
Net carrying amount at March 31, 2019, Intangible assets (excluding MSRs)	$ 5	$ 9
American Airlines, Sears, The Home Depot, Costco and AT&T | Customer Concentration Risk | Intangible Assets, Excluding Mortgage Servicing Rights | Credit card contract related intangibles
Finite and Indefinite-lived Intangible Assets
Concentration risk, percentage	97.00%	97.00%