BUSINESS SEGMENTS (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting [Abstract]
Number of business segments | segment			2
Segment Reporting Information [Line Items]
Revenues, net of interest expense	$ 17,154	$ 17,302	$ 53,955	$ 57,799
Provision (benefits) for income taxes	1,193	777	4,680	1,409
Income (loss) from continuing operations	4,669	3,177	18,839	6,782
Identifiable assets	2,361,876		2,361,876		$ 2,260,090
Operating Segments | Citicorp | North America
Segment Reporting Information [Line Items]
Revenues, net of interest expense	8,500	8,400	25,700	28,300
Operating Segments | Citicorp | EMEA
Segment Reporting Information [Line Items]
Revenues, net of interest expense	3,100	3,100	10,100	9,900
Operating Segments | Citicorp | Latin America
Segment Reporting Information [Line Items]
Revenues, net of interest expense	2,300	2,200	6,700	7,000
Operating Segments | Citicorp | Asia
Segment Reporting Information [Line Items]
Revenues, net of interest expense	3,200	3,800	11,100	12,300
Operating Segments | Global Consumer Banking
Segment Reporting Information [Line Items]
Revenues, net of interest expense	6,260	7,173	20,117	22,686
Provision (benefits) for income taxes	424	284	1,654	(254)
Income (loss) from continuing operations	1,334	920	5,337	(569)
Identifiable assets	442,000		442,000		434,000
Provisions for credit losses and for benefits and claims	(100)	1,700	(400)	10,600
Operating Segments | Institutional Clients Group
Segment Reporting Information [Line Items]
Revenues, net of interest expense	10,786	10,353	33,393	34,974
Provision (benefits) for income taxes	991	800	3,821	2,284
Income (loss) from continuing operations	3,443	2,857	13,210	8,253
Identifiable assets	1,819,000		1,819,000		1,730,000
Provisions for credit losses and for benefits and claims	(0)	800	(2,600)	6,700
Corporate/Other
Segment Reporting Information [Line Items]
Revenues, net of interest expense	108	(224)	445	139
Provision (benefits) for income taxes	(222)	(307)	(795)	(621)
Income (loss) from continuing operations	(108)	(600)	292	(902)
Identifiable assets	101,000		101,000		$ 96,000
Provisions for credit losses and for benefits and claims	$ (100)	$ (100)	$ (300)	$ 200