EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions			3 Months Ended					9 Months Ended
		Oct. 27, 2021	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests				$ 4,669			$ 3,177	$ 18,839	$ 6,782
Noncontrolling interests				24			24	67	18
Net income from continuing operations (for EPS purposes)				4,645			3,153	18,772	6,764
Income (loss) from discontinued operations, net of taxes				(1)			(7)	7	(26)
Citigroup’s net income				4,644			3,146	18,779	6,738
Less: Preferred dividends				266			284	811	828
Net income available to common shareholders				4,378			2,862	17,968	5,910
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with rights to dividends, applicable to basic EPS				26			18	134	50
Net income allocated to common shareholders for basic EPS				$ 4,352			$ 2,844	$ 17,834	$ 5,860
Weighted-average common shares outstanding applicable to basic EPS (in shares)				2,009,300			2,081,800	2,049,300	2,087,100
Basic earnings per share
Income from continuing operations (in dollars per share)	[1]			$ 2.17			$ 1.37	$ 8.70	$ 2.82
Discontinued operations (in dollars per share)	[1]			0			0	0	(0.01)
Net income (in dollars per share)	[1]			$ 2.17			$ 1.37	$ 8.70	$ 2.81
Add back: Dividends allocated to employee restricted and deferred shares with rights to dividends that are forfeitable				$ 8			$ 7	$ 23	$ 22
Net income allocated to common shareholders to diluted EPS				$ 4,360			$ 2,851	$ 17,857	$ 5,882
Effect of dilutive securities
Options (in shares)				0			0	0	0
Other employee plans (in shares)				16,900			12,500	16,000	13,000
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)				2,026,200			2,094,300	2,065,300	2,100,100
Diluted earnings per share
Income from continuing operations (in dollars per share)	[1]			$ 2.15			$ 1.36	$ 8.64	$ 2.81
Discontinued operations (in dollars per share)	[1]			0			0	0	(0.01)
Net income (in dollars per share)	[1]			$ 2.15			$ 1.36	$ 8.65	$ 2.80
Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Issuance of new preferred stock								$ 2,300	$ 1,500
Redemption of preferred stock								$ 3,785	$ 1,500
Preferred stock | Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Preferred dividends declared			$ 228
Series Y | Preferred stock | Subsequent event
Antidilutive Securities Excluded from Computation of Earnings Per Share
Stock issued (in shares)		1,000
Issuance of new preferred stock		$ 1,000
Series Q | Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock (in shares)					1,250
Redemption of preferred stock					$ 1,250
Series R | Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock (in shares)					1,035	465,000
Redemption of preferred stock					$ 1,035	$ 465
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 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

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