EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended					6 Months Ended
		Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests				$ 6,193		$ 1,057	$ 14,170	$ 3,605
Noncontrolling interests				10		0	43	(6)
Net income from continuing operations (for EPS purposes)				6,183		1,057	14,127	3,611
Income (loss) from discontinued operations, net of taxes				10		(1)	8	(19)
Citigroup’s net income				6,193		1,056	14,135	3,592
Less: Preferred dividends				253		253	545	544
Net income available to common shareholders				5,940		803	13,590	3,048
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with rights to dividends, applicable to basic EPS				41		11	107	32
Net income allocated to common shareholders for basic EPS				$ 5,899		$ 792	$ 13,483	$ 3,016
Weighted-average common shares outstanding applicable to basic EPS (in shares)				2,056,500		2,081,700	2,069,300	2,089,800
Basic earnings per share
Income from continuing operations (in dollars per share)	[1]			$ 2.86		$ 0.38	$ 6.51	$ 1.45
Discontinued operations (in dollars per share)	[1]			0		0	0	(0.01)
Net income (in dollars per share)	[1]			$ 2.87		$ 0.38	$ 6.52	$ 1.44
Add back: Dividends allocated to employee restricted and deferred shares with rights to dividends that are forfeitable				$ 8		$ 0	$ 15	$ 15
Net income allocated to common shareholders to diluted EPS				$ 5,907		$ 792	$ 13,498	$ 3,031
Effect of dilutive securities
Options (in shares)				0		0	0	0
Other employee plans (in shares)				16,500		2,600	15,500	13,200
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)				2,073,000		2,084,300	2,084,800	2,103,000
Diluted earnings per share
Income from continuing operations (in dollars per share)	[1]			$ 2.84		$ 0.38	$ 6.47	$ 1.45
Discontinued operations (in dollars per share)	[1]			0		0	0	(0.01)
Net income (in dollars per share)	[1]			$ 2.85		$ 0.38	$ 6.47	$ 1.44
Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Redemption of preferred stock				$ 2,285			$ 3,785	$ 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			$ 266
Distribution of preferred dividends		$ 228
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)				1,035	465,000
Redemption of preferred stock				$ 1,035	$ 465
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
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.