GOODWILL AND INTANGIBLE ASSETS - Changes in Goodwill by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill
Balance of goodwill at beginning of period	$ 21,659	$ 22,349	$ 23,592
Foreign exchange translation and other	729	(613)	(1,000)
Divestitures	(104)	(77)	(212)
Impairment of goodwill	(28)		(31)
Balance of goodwill at end of period	22,256	21,659	22,349
Impairments	28		31
Global Consumer Banking
Goodwill
Balance of goodwill at beginning of period	12,530	12,704
Foreign exchange translation and other	286	(174)
Divestitures	(32)	0
Impairment of goodwill	0
Balance of goodwill at end of period	12,784	12,530	12,704
Impairments	0
Institutional Clients Group
Goodwill
Balance of goodwill at beginning of period	9,085	9,545
Foreign exchange translation and other	443	(447)
Divestitures	(72)	(13)
Impairment of goodwill	0
Balance of goodwill at end of period	9,456	9,085	9,545
Impairments	0
Corporate/Other
Goodwill
Balance of goodwill at beginning of period	44	100
Foreign exchange translation and other	0	8
Divestitures	0	(64)
Impairment of goodwill	(28)
Balance of goodwill at end of period	16	$ 44	100
Impairments	28
Citi Holdings
Goodwill
Impairment of goodwill	(28)
Impairments	$ 28
Consumer Finance South Korea | Citi Holdings
Goodwill
Impairment of goodwill			(16)
Impairments			16
Consumer Latin America | Citi Holdings
Goodwill
Impairment of goodwill			(15)
Impairments			$ 15