SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) - Schedule of Interim Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenues, net of interest expense	$ 16,499	$ 17,302	$ 19,766	$ 20,731	$ 18,378	$ 18,574	$ 18,758	$ 18,576	$ 74,298		$ 74,286		$ 72,854
Operating expenses	11,104	10,964	10,460	10,643	10,454	10,464	10,500	10,584	43,171		42,002		41,841
Provisions (release) for credit losses and for benefits and claims	(46)	2,384	8,197	6,960	2,222	2,088	2,093	1,980	17,495		8,383		7,568
Income from continuing operations before income taxes	5,441	3,954	1,109	3,128	5,702	6,022	6,165	6,012
Provision for income taxes	1,116	777	52	580	703	1,079	1,373	1,275	2,525		4,430		5,357
Income from continuing operations	4,325	3,177	1,057	2,548	4,999	4,943	4,792	4,737	11,107		19,471		18,088
Income (loss) from discontinued operations, net of taxes	6	(7)	(1)	(18)	(4)	(15)	17	(2)	(20)		(4)		(8)
Net income before attribution of noncontrolling interests	4,331	3,170	1,056	2,530	4,995	4,928	4,809	4,735	11,087		19,467		18,080
Noncontrolling interests	22	24	0	(6)	16	15	10	25	40		66		35
Citigroup’s net income	$ 4,309	$ 3,146	$ 1,056	$ 2,536	$ 4,979	$ 4,913	$ 4,799	$ 4,710	$ 11,047		$ 19,401		$ 18,045
Basic
Income from continuing operations (in dollars per share)	$ 1.93	$ 1.37	$ 0.38	$ 1.07	$ 2.16	$ 2.09	$ 1.94	$ 1.88	$ 4.75	[1]	$ 8.08	[1]	$ 6.69	[1]
Net income (in dollars per share)	1.93	1.37	0.38	1.06	2.16	2.09	1.95	1.88	4.74	[1]	8.08	[1]	6.69	[1]
Diluted
Income from continuing operations (in dollars per share)	1.92	1.36	0.38	1.06	2.15	2.08	1.94	1.87	4.73	[1]	8.04	[1]	6.69	[1]
Net income (in dollars per share)	$ 1.92	$ 1.36	$ 0.38	$ 1.06	$ 2.15	$ 2.07	$ 1.95	$ 1.87	$ 4.72	[1]	$ 8.04	[1]	$ 6.68	[1]
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.