EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		1 Months Ended		3 Months Ended				6 Months Ended
		Mar. 31, 2020	Jan. 31, 2020	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests						$ 1,317	$ 4,792	$ 3,851	$ 9,529
Noncontrolling interests						0	10	(6)	35
Net income from continuing operations (for EPS purposes)						1,317	4,782	3,857	9,494
Loss from discontinued operations, net of taxes						(1)	17	(19)	15
Citigroup’s net income						1,316	4,799	3,838	9,509
Less: Preferred dividends						253	296	544	558
Net income available to common shareholders						1,063	4,503	3,294	8,951
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with rights to dividends, applicable to basic EPS						11	50	32	109
Net income allocated to common shareholders for basic EPS						$ 1,052	$ 4,453	$ 3,262	$ 8,842
Weighted-average common shares outstanding applicable to basic EPS (in shares)						2,081.7	2,286.1	2,089.8	2,313.2
Basic earnings per share
Income from continuing operations (in dollars per share)	[1]					$ 0.51	$ 1.94	$ 1.57	$ 3.81
Discontinued operations (in dollars per share)	[1]					0	0.01	(0.01)	0.01
Net income (in dollars per share)	[1]					$ 0.51	$ 1.95	$ 1.56	$ 3.82
Add back: Dividends allocated to employee restricted and deferred shares with rights to dividends that are forfeitable						$ 0	$ 0	$ 14	$ 0
Net income allocated to common shareholders to diluted EPS						$ 1,052	$ 4,453	$ 3,276	$ 8,842
Effect of dilutive securities
Options (in shares)						0.0	0.0	0.0	0.1
Other employee plans (in shares)						2.6	2.9	13.2	2.4
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)						2,084.3	2,289.0	2,103.0	2,315.7
Diluted earnings per share
Income from continuing operations (in dollars per share)	[1]					$ 0.51	$ 1.94	$ 1.57	$ 3.81
Discontinued operations (in dollars per share)	[1]					0	0.01	(0.01)	0.01
Net income (in dollars per share)	[1]					$ 0.50	$ 1.95	$ 1.56	$ 3.82
Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Distribution of preferred dividends				$ 253	$ 284
Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock						$ 0	$ 0	$ 1,500	$ 480
Issuance of new preferred stock						$ 0	$ 0	$ 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.