EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended			9 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests			$ 4,137	$ 3,887	$ 12,138	$ 11,442
Less: Noncontrolling interests from continuing operations			(1)	17	41	48
Net income from continuing operations (for EPS purposes)			4,138	3,870	12,097	11,394
Income (loss) from discontinued operations, net of taxes			(5)	(30)	(2)	(55)
Citigroup’s net income			4,133	3,840	12,095	11,339
Less: Preferred dividends			272	225	893	757
Net income available to common shareholders			3,861	3,615	11,202	10,582
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with nonforfeitable rights to dividends, applicable to basic EPS			53	53	156	145
Net income allocated to common shareholders for basic EPS			3,808	3,562	11,046	10,437
Net income allocated to common shareholders for diluted EPS			$ 3,808	$ 3,562	$ 11,046	$ 10,437
Weighted-average common shares outstanding applicable to basic EPS (in shares)			2,683.6	2,879.9	2,729.3	2,912.9
Effect of dilutive securities
Options (in shares)			0.1	0.1	0.1	0.1
Other employee plans (in shares)			0.0	0.1	0.0	0.1
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)			2,683.7	2,880.1	2,729.5	2,913.0
Basic earnings per share
Income from continuing operations (in dollars per share)	[1]		$ 1.42	$ 1.25	$ 4.05	$ 3.60
Discontinued operations (in dollars per share)	[1]		0.00	(0.01)	0.00	(0.02)
Net income (in dollars per share)	[1]		1.42	1.24	4.05	3.58
Diluted earnings per share
Income from continuing operations (in dollars per share)	[1]		1.42	1.25	4.05	3.60
Discontinued operations (in dollars per share)	[1]		0.00	(0.01)	0.00	(0.02)
Net income (in dollars per share)	[1]		1.42	1.24	4.05	3.58
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)			$ 178.50	$ 178.50	$ 178.50	$ 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 $105.27
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities exercise price (in dollars per share)			$ 105.27	$ 105.27	$ 105.27	$ 105.27
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)			$ 206.70	$ 85.92	$ 206.70	$ 85.92
Antidilutive securities excluded from computation of earnings per common share (in shares)			0.8	3.6
Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Distribution of preferred dividends		$ 320

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