SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenues, net of interest expense	$ 18,378	$ 18,574	$ 18,758	$ 18,576	$ 17,124	$ 18,389	$ 18,469	$ 18,872	$ 74,286		$ 72,854		$ 72,444
Operating expenses	10,454	10,464	10,500	10,584	9,893	10,311	10,712	10,925	42,002		41,841		42,232
Provisions for credit losses and for benefits and claims	2,222	2,088	2,093	1,980	1,925	1,974	1,812	1,857	8,383		7,568		7,451
Income from continuing operations before income taxes	5,702	6,022	6,165	6,012	5,306	6,104	5,945	6,090
Provision for income taxes	703	1,079	1,373	1,275	1,001	1,471	1,444	1,441	4,430		5,357		29,388
Income (loss) from continuing operations	4,999	4,943	4,792	4,737	4,305	4,633	4,501	4,649	19,471		18,088		(6,627)
Income (loss) from discontinued operations, net of taxes	(4)	(15)	17	(2)	(8)	(8)	15	(7)	(4)		(8)		(111)
Net income (loss) before attribution of noncontrolling interests	4,995	4,928	4,809	4,735	4,297	4,625	4,516	4,642	19,467		18,080		(6,738)
Noncontrolling interests	16	15	10	25	(16)	3	26	22	66		35		60
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)
Basic
Income 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]
Net income (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]
Diluted
Income 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)
Net income (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)
Income tax benefit from discrete tax items	$ 540	$ 230
Income tax benefit from reduction in valuation allowance related to deferred tax assets	$ 430	$ 180

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