EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2019	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests		$ 4,792	$ 4,501		$ 9,529	$ 9,150
Noncontrolling interests		10	26		35	48
Net income from continuing operations (for EPS purposes)		4,782	4,475		9,494	9,102
Loss from discontinued operations, net of taxes		17	15		15	8
Citigroup’s net income		4,799	4,490		9,509	9,110
Less: Preferred dividends		296	318		558	590
Net income available to common shareholders		4,503	4,172		8,951	8,520
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with nonforfeitable rights to dividends, applicable to basic EPS		50	49		109	90
Net income allocated to common shareholders for basic EPS		4,453	4,123		8,842	8,430
Net income allocated to common shareholders to diluted EPS		$ 4,453	$ 4,123		$ 8,842	$ 8,430
Weighted-average common shares outstanding applicable to basic EPS (in shares)		2,286.1	2,530.9		2,313.2	2,546.2
Effect of dilutive securities
Options (in shares)		0.0	0.1		0.1	0.1
Other employee plans (in shares)		2.9	1.3		2.4	1.3
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)		2,289.0	2,532.3		2,315.7	2,547.6
Basic earnings per share
Income from continuing operations (in dollars per share)	[1]	$ 1.94	$ 1.62		$ 3.81	$ 3.30
Discontinued operations (in dollars per share)	[1]	0.01	0.01		0.01	0.01
Net income (in dollars per share)	[1]	1.95	1.63		3.82	3.31
Diluted earnings per share
Income from continuing operations (in dollars per share)	[1]	1.94	1.62		3.81	3.30
Discontinued operations (in dollars per share)	[1]	0.01	0.01		0.01	0.01
Net income (in dollars per share)	[1]	$ 1.95	$ 1.63		$ 3.82	3.31
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)			0.5
Antidilutive securities exercise price (in dollars per share)			$ 148.77			$ 148.77
Forecast
Antidilutive Securities Excluded from Computation of Earnings Per Share
Distribution of preferred dividends				$ 550

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