EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		1 Months Ended		3 Months Ended
		Mar. 31, 2020	Jan. 31, 2020	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests							$ 2,534	$ 4,737
Noncontrolling interests							(6)	25
Net income from continuing operations (for EPS purposes)							2,540	4,712
Loss from discontinued operations, net of taxes							(18)	(2)
Citigroup’s net income							2,522	4,710
Less: Preferred dividends							291	262
Net income available to common shareholders							2,231	4,448
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with rights to dividends, applicable to basic EPS							21	59
Net income allocated to common shareholders for basic EPS							$ 2,210	$ 4,389
Weighted-average common shares outstanding applicable to basic EPS (in shares)							2,097.9	2,340.4
Basic earnings per share
Income from continuing operations (in dollars per share)	[1]						$ 1.06	$ 1.88
Discontinued operations (in dollars per share)	[1]						(0.01)	0
Net income (in dollars per share)	[1]						$ 1.05	$ 1.88
Add back: Dividends allocated to employee restricted and deferred shares with rights to dividends that are forfeitable							$ 7	$ 0
Net income allocated to common shareholders to diluted EPS							$ 2,217	$ 4,389
Effect of dilutive securities
Options (in shares)							0.1	0.1
Other employee plans (in shares)							15.7	1.9
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)							2,113.7	2,342.4
Diluted earnings per share
Income from continuing operations (in dollars per share)	[1]						$ 1.06	$ 1.87
Discontinued operations (in dollars per share)	[1]						(0.01)	0
Net income (in dollars per share)	[1]						$ 1.05	$ 1.87
Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Distribution of preferred dividends				$ 253	$ 284	$ 253
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	$ 0
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.