SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ 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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues, net of interest expense	$ 17,255	$ 18,173	$ 17,901	$ 18,120	$ 17,012	$ 17,760	$ 17,548	$ 17,555					$ 71,449	[1]	$ 69,875	[1]	$ 76,354	[1]
Operating expenses	10,083	10,171	10,506	10,477	10,120	10,404	10,369	10,523					41,237	[1]	41,416	[1]	43,615	[1]
Provisions for credit losses and for benefits and claims	2,073	1,999	1,717	1,662	1,792	1,736	1,409	2,045					7,451		6,982		7,913
Income from continuing operations before income taxes	5,099	6,003	5,678	5,981	5,100	5,620	5,770	4,987
Provision for income taxes (benefits)	23,864	1,866	1,795	1,863	1,509	1,733	1,723	1,479					29,388		6,444		7,440
Income (loss) from continuing operations	(18,765)	4,137	3,883	4,118	3,591	3,887	4,047	3,508					(6,627)		15,033		17,386
Income (loss) from discontinued operations, net of taxes	(109)	(5)	21	(18)	(3)	(30)	(23)	(2)					(111)		(58)		(54)
Net income (loss) before attribution of noncontrolling interests	(18,874)	4,132	3,904	4,100	3,588	3,857	4,024	3,506					(6,738)		14,975		17,332
Noncontrolling interests	19	(1)	32	10	15	17	26	5					60		63		90
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
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)	[2]	$ 4.74	[2]	$ 5.43	[2]
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)	[2]	4.72	[2]	5.41	[2]
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)	[2]	4.74	[2]	5.42	[2]
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)	[2]	4.72	[2]	$ 5.40	[2]
Common stock price per share
High (in dollars per share)	77.10	72.74	66.98	61.54	61.09	47.90	47.33	51.13					77.10		61.09
Low (in dollars per share)	71.33	65.95	57.72	55.68	47.03	40.78	38.48	34.98					71.33		47.03
Close (in dollars per share)	74.41	72.74	66.88	59.82	59.43	47.23	42.39	41.75					$ 74.41		$ 59.43
Dividends per share of common stock (in dollars per share)	$ 0.32	$ 0.32	$ 0.16	$ 0.16	$ 0.16	$ 0.16	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.01

[1] Certain prior-period revenue and expense lines and totals were reclassified to conform to the current period’s presentation. See Note 3 to the Consolidated Financial Statements.

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