EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		1 Months Ended		3 Months Ended			9 Months Ended
		Mar. 31, 2020	Jan. 31, 2020	Dec. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests					$ 3,261	$ 4,943	$ 7,112	$ 14,472
Noncontrolling interests					24	15	18	50
Net income from continuing operations (for EPS purposes)					3,237	4,928	7,094	14,422
Loss from discontinued operations, net of taxes					(7)	(15)	(26)	0
Citigroup’s net income					3,230	4,913	7,068	14,422
Less: Preferred dividends					284	254	828	812
Net income available to common shareholders					2,946	4,659	6,240	13,610
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with rights to dividends, applicable to basic EPS					18	27	50	88
Net income allocated to common shareholders for basic EPS					$ 2,928	$ 4,632	$ 6,190	$ 13,522
Weighted-average common shares outstanding applicable to basic EPS (in shares)					2,081.8	2,220.8	2,087.1	2,282.4
Basic earnings per share
Income from continuing operations (in dollars per share)	[1]				$ 1.41	$ 2.09	$ 2.98	$ 5.92
Discontinued operations (in dollars per share)	[1]				0	(0.01)	(0.01)	0
Net income (in dollars per share)	[1]				$ 1.41	$ 2.09	$ 2.97	$ 5.92
Add back: Dividends allocated to employee restricted and deferred shares with rights to dividends that are forfeitable					$ 7	$ 9	$ 21	$ 24
Net income allocated to common shareholders to diluted EPS					$ 2,935	$ 4,641	$ 6,211	$ 13,546
Effect of dilutive securities
Options (in shares)					0.0	0.1	0.0	0.1
Other employee plans (in shares)					12.5	16.2	13.0	15.7
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)					2,094.3	2,237.1	2,100.1	2,298.2
Diluted earnings per share
Income from continuing operations (in dollars per share)	[1]				$ 1.40	$ 2.08	$ 2.97	$ 5.89
Discontinued operations (in dollars per share)	[1]				0	(0.01)	(0.01)	0
Net income (in dollars per share)	[1]				$ 1.40	$ 2.07	2.96	$ 5.89
Weighted-average options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings per common share (in shares)					0.1
Antidilutive securities exercise price (in dollars per share)					$ 56.25		$ 56.25
Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Distribution of preferred dividends				$ 267
Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock							$ 1,500	$ 480
Issuance of new preferred stock						$ 1,500	$ 1,500	$ 1,500
Series O | Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock (in shares)		1.5
Redemption of preferred stock		$ 1,500
Series V | Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Stock issued (in shares)			1.5
Issuance of new preferred stock			$ 1,500

[1] Due to rounding, earnings per share on continuing operations and discontinued operations may not sum to earnings per share on net income.