GOODWILL AND INTANGIBLE ASSETS - Changes in Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	$ 4,588
Acquisitions/ divestitures	0
Amortization	(135)
FX translation and other	(3)
Net carrying amount at March 31, 2018, Intangible assets (excluding MSRs)	4,450	$ 4,588
Mortgage servicing rights (MSRs)	587	558
Total intangible assets	5,037	5,146
Indefinite-lived intangible assets
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	244
Acquisitions/ divestitures	0
Amortization	0
FX translation and other	(13)
Net carrying amount at March 31, 2018, Intangible assets (excluding MSRs)	231	244
Purchased credit card relationships
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	1,539
Acquisitions/ divestitures	0
Amortization	(35)
FX translation and other	(1)
Net carrying amount at March 31, 2018, Intangible assets (excluding MSRs)	1,503	1,539
Credit card contract related intangibles
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	2,589
Acquisitions/ divestitures	0
Amortization	(86)
FX translation and other	(1)
Net carrying amount at March 31, 2018, Intangible assets (excluding MSRs)	2,502	2,589
Core deposit intangibles
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	11
Acquisitions/ divestitures	0
Amortization	(2)
FX translation and other	1
Net carrying amount at March 31, 2018, Intangible assets (excluding MSRs)	10	11
Other customer relationships
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	187
Acquisitions/ divestitures	0
Amortization	(6)
FX translation and other	11
Net carrying amount at March 31, 2018, Intangible assets (excluding MSRs)	192	187
Present value of future profits
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	4
Acquisitions/ divestitures	0
Amortization	0
FX translation and other	0
Net carrying amount at March 31, 2018, Intangible assets (excluding MSRs)	4	4
Other
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	14
Acquisitions/ divestitures	0
Amortization	(6)
FX translation and other	0
Net carrying amount at March 31, 2018, Intangible assets (excluding MSRs)	$ 8	$ 14
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%