EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Earnings Per Share [Abstract]
Income (loss) from continuing operations before attribution of noncontrolling interests		$ 4,305	$ 4,633	$ 4,501	$ 4,649	$ (18,765)	$ 4,137	$ 3,883	$ 4,118	$ 18,088		$ (6,627)		$ 15,033
Noncontrolling interests		(16)	3	26	22	19	(1)	32	10	35		60		63
Net income (loss) from continuing operations (for EPS purposes)										18,053		(6,687)		14,970
Income (loss) from discontinued operations, net of taxes		(8)	(8)	15	(7)	(109)	(5)	21	(18)	(8)		(111)		(58)
Citigroup’s net income (loss)		$ 4,313	$ 4,622	$ 4,490	$ 4,620	$ (18,893)	$ 4,133	$ 3,872	$ 4,090	18,045		(6,798)		14,912
Less: Preferred dividends										1,173		1,213		1,077
Net income (loss) available to common shareholders										16,872		(8,011)		13,835
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with nonforfeitable rights to dividends, applicable to basic EPS										200		37		195
Net income (loss) allocated to common shareholders for basic and diluted EPS										$ 16,672		$ (8,048)		$ 13,640
Weighted-average common shares outstanding applicable to basic EPS (in shares)										2,493.3		2,698.5		2,888.1
Effect of dilutive securities
Options (in shares)										0.1		0.0		0.1
Other employee plans (in shares)										1.4		0.0		0.1
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)										2,494.8		2,698.5		2,888.3
Basic earnings per share
Income (loss) from continuing operations (in dollars per share)		$ 1.65	$ 1.74	$ 1.62	$ 1.68	$ (7.33)	$ 1.42	$ 1.27	$ 1.36	$ 6.69	[1]	$ (2.94)	[1]	$ 4.74	[1]
Discontinued operations (in dollars per share)	[1]									0.00		(0.04)		(0.02)
Net income (loss) (in dollars per share)		1.65	1.73	1.63	1.68	(7.38)	1.42	1.28	1.35	6.69	[1]	(2.98)	[1]	4.72	[1]
Diluted earnings per share
Income (loss) from continuing operations (in dollars per share)		1.65	1.74	1.62	1.68	(7.33)	1.42	1.27	1.36	6.69	[1]	(2.94)	[1]	4.74	[1]
Discontinued operations (in dollars per share)	[1]									0.00		(0.04)		(0.02)
Net income (loss) (in dollars per share)		1.64	$ 1.73	$ 1.63	$ 1.68	(7.38)	$ 1.42	$ 1.28	$ 1.35	$ 6.68	[1]	$ (2.98)	[1]	$ 4.72	[1]
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		4.2
Antidilutive securities exercise price (in dollars per share)		$ 145.69				$ 204.80				$ 145.69		$ 204.80		$ 98.01

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