SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenues, net of interest expense	$ 17,124	$ 18,389	$ 18,469	$ 18,872	$ 17,504	$ 18,419	$ 18,155	$ 18,366	$ 72,854	[1]	$ 72,444	[1]	$ 70,797	[1]
Operating expenses	9,893	10,311	10,712	10,925	10,332	10,417	10,760	10,723	41,841	[1]	42,232	[1]	42,338	[1]
Provisions for credit losses and for benefits and claims	1,925	1,974	1,812	1,857	2,073	1,999	1,717	1,662	7,568		7,451		6,982
Income from continuing operations before income taxes	5,306	6,104	5,945	6,090	5,099	6,003	5,678	5,981
Provision for income taxes	1,001	1,471	1,444	1,441	23,864	1,866	1,795	1,863	5,357		29,388		6,444
Income (loss) from continuing operations	4,305	4,633	4,501	4,649	(18,765)	4,137	3,883	4,118	18,088		(6,627)		15,033
Income (loss) from discontinued operations, net of taxes	(8)	(8)	15	(7)	(109)	(5)	21	(18)	(8)		(111)		(58)
Net income (loss) before attribution of noncontrolling interests	4,297	4,625	4,516	4,642	(18,874)	4,132	3,904	4,100	18,080		(6,738)		14,975
Noncontrolling interests	(16)	3	26	22	19	(1)	32	10	35		60		63
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
Basic
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	[2]	$ (2.94)	[2]	$ 4.74	[2]
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	[2]	(2.98)	[2]	4.72	[2]
Diluted
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	[2]	(2.94)	[2]	4.74	[2]
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	[2]	$ (2.98)	[2]	$ 4.72	[2]

[1] Certain prior-period revenue and expense lines and totals were reclassified to conform to the current period’s presentation. See Notes 1 and 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.