EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Earnings Per Share [Abstract]
Income from continuing operations before attribution of noncontrolling interests		$ 4,999	$ 4,943	$ 4,792	$ 4,737	$ 4,305	$ 4,633	$ 4,501	$ 4,649	$ 19,471		$ 18,088		$ (6,627)
Noncontrolling interests		16	15	10	25	(16)	3	26	22	66		35		60
Net income from continuing operations (for EPS purposes)										19,405		18,053		(6,687)
Loss from discontinued operations, net of taxes		(4)	(15)	17	(2)	(8)	(8)	15	(7)	(4)		(8)		(111)
Citigroup’s net income (loss)		$ 4,979	$ 4,913	$ 4,799	$ 4,710	$ 4,313	$ 4,622	$ 4,490	$ 4,620	19,401		18,045		(6,798)
Less: Preferred dividends										1,109		1,174		1,213
Net income available to common shareholders										18,292		16,871		(8,011)
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with rights to dividends, applicable to basic EPS										121		200		37
Net income allocated to common shareholders for basic EPS										$ 18,171		$ 16,671		$ (8,048)
Weighted-average common shares outstanding applicable to basic EPS (in shares)										2,249.2		2,493.3		2,698.5
Basic earnings per share
Income (loss) from continuing operations (in dollars per share)		$ 2.16	$ 2.09	$ 1.94	$ 1.88	$ 1.65	$ 1.74	$ 1.62	$ 1.68	$ 8.08	[1]	$ 6.69	[1]	$ (2.94)	[1]
Discontinued operations (in dollars per share)	[1]									0		0		(0.04)
Net income (loss) (in dollars per share)		2.16	2.09	1.95	1.88	1.65	1.73	1.63	1.68	$ 8.08	[1]	$ 6.69	[1]	$ (2.98)	[1]
Add back: Dividends allocated to employee restricted and deferred shares with rights to dividends that are forfeitable										$ 33		$ 0		$ 0
Net income allocated to common shareholders for diluted EPS										$ 18,204		$ 16,671		$ (8,048)
Effect of dilutive securities
Options (in shares)										0.1		0.1		0.0
Other employee plans (in shares)										16.0		1.4		0.0
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)										2,265.3		2,494.8		2,698.5
Diluted earnings per share
Income (loss) from continuing operations (in dollars per share)		2.15	2.08	1.94	1.87	1.65	1.74	1.62	1.68	$ 8.04		$ 6.69		$ (2.94)
Discontinued operations (in dollars per share)										0		0		(0.04)
Net income (loss) (in dollars per share)		$ 2.15	$ 2.07	$ 1.95	$ 1.87	1.64	$ 1.73	$ 1.63	$ 1.68	$ 8.04		$ 6.68		$ (2.98)
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		0.8
Antidilutive securities exercise price (in dollars per share)						$ 145.69						$ 145.69		$ 204.80

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