BUSINESS SEGMENTS (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment reporting information
Revenues, net of interest expense	$ 18,389	$ 18,419	$ 55,730	$ 54,940
Provision (benefits) for income taxes	1,471	1,866	4,356	5,524
Income (loss) from continuing operations	4,633	4,137	13,783	12,138
Identifiable assets	1,925,165		1,925,165		$ 1,842,465
Provisions for credit losses and for benefits and claims	1,974	1,999	5,643	5,378
Operating Segments | Citicorp | North America
Segment reporting information
Revenues, net of interest expense	8,500	8,900	25,400	26,000
Operating Segments | Citicorp | EMEA
Segment reporting information
Revenues, net of interest expense	2,900	2,700	9,100	8,400
Operating Segments | Citicorp | Latin America
Segment reporting information
Revenues, net of interest expense	2,700	2,500	7,800	7,200
Operating Segments | Citicorp | Asia
Segment reporting information
Revenues, net of interest expense	3,800	3,800	11,800	10,900
Operating Segments | Global Consumer Banking
Segment reporting information
Revenues, net of interest expense	8,654	8,470	25,337	24,389
Provision (benefits) for income taxes	493	635	1,357	1,863
Income (loss) from continuing operations	1,567	1,170	4,240	3,296
Identifiable assets	427,000		427,000		428,000
Provisions for credit losses and for benefits and claims	1,900	2,200	5,700	5,800
Operating Segments | Institutional Clients Group
Segment reporting information
Revenues, net of interest expense	9,241	9,430	28,780	28,170
Provision (benefits) for income taxes	862	1,394	2,890	4,096
Income (loss) from continuing operations	3,117	3,062	9,683	8,853
Identifiable assets	1,404,000		1,404,000		1,336,000
Provisions for credit losses and for benefits and claims	71	(164)	55	(282)
Corporate/Other
Segment reporting information
Revenues, net of interest expense	494	519	1,613	2,381
Provision (benefits) for income taxes	116	(163)	109	(435)
Income (loss) from continuing operations	(51)	(95)	(140)	(11)
Identifiable assets	94,000		94,000		$ 78,000
Provisions for credit losses and for benefits and claims	$ (30)	$ (50)	$ (155)	$ (130)