EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended
	Oct. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests			$ 4,633.0	$ 4,137.0	$ 13,783.0	$ 12,138.0
Less: Noncontrolling interests from continuing operations			3.0	(1.0)	51.0	41.0
Net income from continuing operations (for EPS purposes)			4,630.0	4,138.0	13,732.0	12,097.0
Loss from discontinued operations, net of taxes			(8.0)	(5.0)	0.0	(2.0)
Citigroup’s net income			4,622.0	4,133.0	13,732.0	12,095.0
Less: Preferred dividends			270.0	272.0	860.0	893.0
Net income available to common shareholders			4,352.0	3,861.0	12,872.0	11,202.0
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with nonforfeitable rights to dividends, applicable to basic EPS			51.0	53.0	151.0	156.0
Net income allocated to common shareholders for basic EPS			4,301.0	3,808.0	12,721.0	11,046.0
Net income allocated to common shareholders for diluted EPS			$ 4,301.0	$ 3,808.0	$ 12,721.0	$ 11,046.0
Weighted-average common shares outstanding applicable to basic EPS (in shares)			2,479.8	2,683.6	2,524.1	2,729.3
Effect of dilutive securities
Options (in shares)			0.2	0.1	0.1	0.1
Other employee plans (in shares)			1.4	0.0	1.3	0.0
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)			2,481.4	2,683.7	2,525.5	2,729.5
Basic earnings per share
Income from continuing operations (in dollars per share)			$ 1.74	$ 1.42	$ 5.04	$ 4.05
Discontinued operations (in dollars per share)			0.00	0.00	0.00	0.00
Net income (in dollars per share)			1.73	1.42	5.04	4.05
Diluted earnings per share
Income from continuing operations (in dollars per share)			1.74	1.42	5.04	4.05
Discontinued operations (in dollars per share)			0.00	0.00	0.00	0.00
Net income (in dollars per share)			1.73	1.42	5.04	4.05
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 $103.82
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities exercise price (in dollars per share)			$ 103.82	$ 103.82	$ 103.82	$ 103.82
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)			$ 142.30	$ 206.70	$ 142.30	$ 206.70
Antidilutive securities excluded from computation of earnings per common share (in shares)			0.5	0.8
Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Distribution of preferred dividends		$ 313.0
Preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Treasury stock acquired			$ 0.0		$ 218.0	$ 0.0
Preferred stock | Series AA
Antidilutive Securities Excluded from Computation of Earnings Per Share
Number of shares redeemed					3.8
Treasury stock acquired					$ 96.8
Preferred stock | Series E
Antidilutive Securities Excluded from Computation of Earnings Per Share
Number of shares redeemed					4.9
Treasury stock acquired					$ 121.3
Subsequent event | Preferred stock | Series C
Antidilutive Securities Excluded from Computation of Earnings Per Share
Number of shares redeemed	23.0
Treasury stock acquired	$ 575.0