EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions			1 Months Ended	3 Months Ended
		Apr. 16, 2021	Feb. 28, 2021	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests							$ 7,977	$ 2,548
Noncontrolling interests							33	(6)
Net income from continuing operations (for EPS purposes)							7,944	2,554
Loss from discontinued operations, net of taxes							(2)	(18)
Citigroup’s net income							7,942	2,536
Less: Preferred dividends							292	291
Net income available to common shareholders							7,650	2,245
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with rights to dividends, applicable to basic EPS							66	21
Net income allocated to common shareholders for basic EPS							$ 7,584	$ 2,224
Weighted-average common shares outstanding applicable to basic EPS (in shares)							2,082,000	2,097,900
Basic earnings per share
Income from continuing operations (in dollars per share)	[1]						$ 3.64	$ 1.07
Discontinued operations (in dollars per share)	[1]						0	(0.01)
Net income (in dollars per share)	[1]						$ 3.64	$ 1.06
Add back: Dividends allocated to employee restricted and deferred shares with rights to dividends that are forfeitable							$ 7	$ 7
Net income allocated to common shareholders to diluted EPS							$ 7,591	$ 2,231
Effect of dilutive securities
Options (in shares)							100	100
Other employee plans (in shares)							14,500	15,700
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)							2,096,600	2,113,700
Diluted earnings per share
Income from continuing operations (in dollars per share)	[1]						$ 3.62	$ 1.06
Discontinued operations (in dollars per share)	[1]						0	(0.01)
Net income (in dollars per share)	[1]						$ 3.62	$ 1.06
Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock							$ 1,500	$ 1,500
Issuance of new preferred stock							$ 2,300	$ 1,500
Preferred stock | Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Preferred dividends declared						$ 253
Distribution of preferred dividends				$ 228	$ 266
Series S | Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock (in shares)							41,400
Redemption of preferred stock							$ 1,035
Series R | Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock (in shares)							465,000
Redemption of preferred stock							$ 465
Series R | Preferred stock | Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock (in shares)		1,035
Redemption of preferred stock		$ 1,035
Series X | Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Stock issued (in shares)			2,300
Issuance of new preferred stock			$ 2,300
Series Q | Preferred stock | Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock (in shares)		1,250
Redemption of preferred stock		$ 1,250

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