EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests		$ 3,883	$ 4,047		$ 8,001	$ 7,555
Less: Noncontrolling interests from continuing operations		32	26		42	31
Net income from continuing operations (for EPS purposes)		3,851	4,021		7,959	7,524
Income (loss) from discontinued operations, net of taxes		21	(23)		3	(25)
Citigroup’s net income		3,872	3,998		7,962	7,499
Less: Preferred dividends		320	322		621	532
Net income available to common shareholders		3,552	3,676		7,341	6,967
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with nonforfeitable rights to dividends, applicable to basic EPS		48	53		103	93
Net income allocated to common shareholders for basic EPS		3,504	3,623		7,238	6,874
Net income allocated to common shareholders for diluted EPS		$ 3,504	$ 3,623		$ 7,238	$ 6,874
Weighted-average common shares outstanding applicable to basic EPS (in shares)		2,739.1	2,915.8		2,752.2	2,929.4
Effect of dilutive securities
Options (in shares)		0.1	0.1		0.1	0.1
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)		2,739.2	2,915.9		2,752.3	2,929.5
Basic earnings per share
Income from continuing operations (in dollars per share)	[1]	$ 1.27	$ 1.25		$ 2.63	$ 2.36
Discontinued operations (in dollars per share)	[1]	0.01	(0.01)		0.00	(0.01)
Net income (in dollars per share)	[1]	1.28	1.24		2.63	2.35
Diluted earnings per share
Income from continuing operations (in dollars per share)	[1]	1.27	1.25		2.63	2.36
Discontinued operations (in dollars per share)	[1]	0.01	(0.01)		0.00	(0.01)
Net income (in dollars per share)	[1]	1.28	1.24		2.63	2.35
Warrants issued to U.S. Treasury as part of TARP and loss-sharing agreement | Warrant with the exercise price of $178.50
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities exercise price (in dollars per share)		$ 178.5	$ 178.50		$ 178.5	$ 178.50
Antidilutive securities excluded from computation of earnings per common share (in shares)		21.0	21.0		21.0	21.0
Warrants issued to U.S. Treasury as part of TARP and loss-sharing agreement | Warrant with the exercise price of $105.71
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities exercise price (in dollars per share)		$ 105.61	$ 105.61		$ 105.61	$ 105.61
Antidilutive securities excluded from computation of earnings per common share (in shares)		25.5	25.5		25.5	25.5
Weighted-average options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities exercise price (in dollars per share)		$ 204.80	$ 75.43		$ 204.80	$ 75.43
Antidilutive securities excluded from computation of earnings per common share (in shares)		0.8	5.3
Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Distribution of preferred dividends				$ 592

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