EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2017
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests		$ 4,118	$ 3,508
Less: Noncontrolling interests from continuing operations		10	5
Net income from continuing operations (for EPS purposes)		4,108	3,503
Income (loss) from discontinued operations, net of taxes		(18)	(2)
Citigroup’s net income		4,090	3,501
Less: Preferred dividends		301	210
Net income available to common shareholders		3,789	3,291
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with nonforfeitable rights to dividends, applicable to basic EPS		55	40
Net income allocated to common shareholders for basic EPS		3,734	3,251
Net income allocated to common shareholders for diluted EPS		$ 3,734	$ 3,251
Weighted-average common shares outstanding applicable to basic EPS (in shares)		2,765.3	2,943.0
Effect of dilutive securities
Options (in shares)		0.2	0.1
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)		2,765.5	2,943.1
Basic earnings per share
Income from continuing operations (in dollars per share)	[1]	$ 1.36	$ 1.11
Discontinued operations (in dollars per share)	[1]	(0.01)	0.00
Net income (in dollars per share)	[1]	1.35	1.10
Diluted earnings per share
Income from continuing operations (in dollars per share)		1.36	1.11
Discontinued operations (in dollars per share)		(0.01)	0.00
Net income (in dollars per share)		$ 1.35	$ 1.10
Antidilutive Securities Excluded from Computation of Earnings Per Share
Distribution of preferred dividends		$ 301
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 excluded from computation of earnings per common share (in shares)		21.0	21.0
Antidilutive securities exercise price (in dollars per share)		$ 178.5	$ 178.50
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 excluded from computation of earnings per common share (in shares)		25.5	25.5
Antidilutive securities exercise price (in dollars per share)		$ 105.71	$ 105.71
Weighted-average options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings per common share (in shares)		0.9	6.2
Antidilutive securities exercise price (in dollars per share)		$ 201.01	$ 69.88
Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Distribution of preferred dividends				$ 912

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