EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Earnings Per Share [Abstract]
Income (loss) from continuing operations before attribution of noncontrolling interests		$ (18,765)	$ 4,137	$ 3,883	$ 4,118	$ 3,591	$ 3,887	$ 4,047	$ 3,508	$ (6,627)		$ 15,033		$ 17,386
Less: Noncontrolling interests from continuing operations										60		63		90
Net income (loss) from continuing operations (for EPS purposes)										(6,687)		14,970		17,296
Income (loss) from discontinued operations, net of taxes		(109)	(5)	21	(18)	(3)	(30)	(23)	(2)	(111)		(58)		(54)
Citigroup’s net income (loss)		$ (18,893)	$ 4,133	$ 3,872	$ 4,090	$ 3,573	$ 3,840	$ 3,998	$ 3,501	(6,798)		14,912		17,242
Less: Preferred dividends										1,213		1,077		769
Net income (loss) available to common shareholders										(8,011)		13,835		16,473
Less: Dividends and undistributed earnings allocated to employee restricted and deferred shares with nonforfeitable rights to dividends, applicable to basic EPS										37		195		224
Net income (loss) allocated to common shareholders for basic EPS										(8,048)		13,640		16,249
Add: Interest expense, net of tax, and dividends on convertible securities and adjustment of undistributed earnings allocated to employee restricted and deferred shares with nonforfeitable rights to dividends, applicable to diluted EPS										0		0		0
Net income (loss) allocated to common shareholders for diluted EPS										$ (8,048)		$ 13,640		$ 16,249
Weighted-average common shares outstanding applicable to basic EPS (in shares)										2,698.5		2,888.1		3,004.0
Effect of dilutive securities
Options (in shares)										0.0		0.1		3.6
Other employee plans non-dividend eligible (in shares)										0.0		0.1		0.1
Adjusted weighted-average common shares outstanding applicable to diluted EPS (in shares)										2,698.5		2,888.3		3,007.7
Basic
Income (loss) from continuing operations (in dollars per share)		$ (7.33)	$ 1.42	$ 1.27	$ 1.36	$ 1.14	$ 1.25	$ 1.25	$ 1.11	$ (2.94)	[1]	$ 4.74	[1]	$ 5.43	[1]
Discontinued operations (in dollars per share)	[1]									(0.04)		(0.02)		(0.02)
Net income (loss) (in dollars per share)		(7.38)	1.42	1.28	1.35	1.14	1.24	1.24	1.10	(2.98)	[1]	4.72	[1]	5.41	[1]
Diluted
Income (loss) from continuing operations (in dollars per share)		(7.33)	1.42	1.27	1.36	1.14	1.25	1.25	1.11	(2.94)	[1]	4.74	[1]	5.42	[1]
Discontinued operations (in dollars per share)	[1]									(0.04)		(0.02)		(0.02)
Net income (loss) (in dollars per share)		(7.38)	$ 1.42	$ 1.28	$ 1.35	1.14	$ 1.24	$ 1.24	$ 1.10	(2.98)	[1]	4.72	[1]	5.40	[1]
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)		178.5				178.50				$ 178.5		$ 178.50		$ 178.50
Antidilutive securities excluded from computation of earnings per common share (in shares)										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 $104.96
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities exercise price (in dollars per share)		104.96				104.96				$ 104.96		$ 104.96		$ 104.96
Antidilutive securities excluded from computation of earnings per common share (in shares)										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)		$ 204.80				$ 98.01				$ 204.80		$ 98.01		$ 199.16
Antidilutive securities excluded from computation of earnings per common share (in shares)										0.8		4.2		0.9

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