BUSINESS SEGMENTS (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) country	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) country	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment reporting information
Increase (decrease) in end-of-period loans	$ 699,483				$ 684,196				$ 699,483	$ 684,196
Increase (decrease) in end-of-period deposits	1,070,590				1,013,170				1,070,590	1,013,170
Revenues, net of interest expense	18,378	$ 18,574	$ 18,758	$ 18,576	17,124	$ 18,389	$ 18,469	$ 18,872	74,286	72,854	$ 72,444
Provision (benefits) for income taxes	703	1,079	1,373	1,275	1,001	1,471	1,444	1,441	4,430	5,357	29,388
Income (loss) from continuing operations	4,999	$ 4,943	$ 4,792	$ 4,737	4,305	$ 4,633	$ 4,501	$ 4,649	19,471	18,088	(6,627)
Identifiable assets	1,951,158				1,917,383				1,951,158	1,917,383
Operating Segments | Citicorp | North America
Segment reporting information
Revenues, net of interest expense									33,900	33,400	34,100
Operating Segments | Citicorp | EMEA
Segment reporting information
Revenues, net of interest expense									12,000	11,800	10,900
Operating Segments | Citicorp | Latin America
Segment reporting information
Revenues, net of interest expense									10,400	10,300	9,600
Operating Segments | Citicorp | Asia
Segment reporting information
Revenues, net of interest expense									16,000	15,300	14,600
Operating Segments | Global Consumer Banking
Segment reporting information
Revenues, net of interest expense									32,971	32,339	31,445
Provision (benefits) for income taxes									1,746	1,689	3,067
Income (loss) from continuing operations									5,702	5,309	3,542
Identifiable assets	$ 407,000				388,000				407,000	388,000
Provision for credit losses									$ 7,900	7,600	7,600
Operating Segments | Institutional Clients Group
Segment reporting information
Number of countries where the entity provides a broad range of banking and financial products and services (over) | country	98								98
Revenues, net of interest expense									$ 39,301	38,325	37,822
Provision (benefits) for income taxes									3,570	3,756	7,241
Income (loss) from continuing operations									12,944	12,574	9,375
Identifiable assets	$ 1,447,000				1,438,000				1,447,000	1,438,000
Provision for credit losses									563	215	19
Corporate/Other
Segment reporting information
Revenues, net of interest expense									2,014	2,190	3,177
Provision (benefits) for income taxes									(886)	(88)	19,080
Income (loss) from continuing operations									825	205	(19,544)
Identifiable assets	97,000				91,000				97,000	91,000
Provision for credit losses									(75)	(202)	(175)
Reclassification Adjustment | Operating Segments | Global Consumer Banking
Segment reporting information
Increase (decrease) in end-of-period loans	(28,000)				(28,000)				(28,000)	(28,000)	(28,000)
Increase (decrease) in end-of-period deposits	(40,000)				(40,000)				(40,000)	(40,000)	(40,000)
Reclassification Adjustment | Operating Segments | Institutional Clients Group
Segment reporting information
Increase (decrease) in end-of-period loans	28,000				28,000				28,000	28,000	28,000
Increase (decrease) in end-of-period deposits	$ 37,000				$ 40,000				$ 37,000	$ 40,000	$ 40,000