CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statements of Income and Comprehensive Income (Details) - USD ($) $ 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
Revenues
Dividends from subsidiaries									$ 0	$ 0	$ 0
Interest revenue									58,089	76,510	70,828
Interest revenue—intercompany									0	0	0
Interest expense									14,541	29,163	24,266
Interest expense—intercompany									0	0	0
Net interest revenue									43,548	47,347	46,562
Commissions and fees									11,385	11,746	11,857
Commissions and fees—intercompany									0	0	0
Principal transactions									13,885	8,892	8,905
Principal transactions—intercompany									0	0	0
Other revenue									5,480	6,301	5,530
Other revenue—intercompany									0	0	0
Total non-interest revenues									30,750	26,939	26,292
Total 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
Provisions 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
Operating expenses
Compensation and benefits									22,214	21,433	21,154
Compensation and benefits—intercompany									0	0	0
Other operating									20,957	20,569	20,687
Other operating—intercompany									0	0	0
Total operating expenses	11,104	10,964	10,460	10,643	10,454	10,464	10,500	10,584	43,171	42,002	41,841
Equity in undistributed income of subsidiaries									0	0	0
Income from continuing operations before income taxes									13,632	23,901	23,445
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
Comprehensive income
Add: Other comprehensive income (loss)									4,260	852	(2,499)
Citigroup’s total comprehensive income									15,307	20,253	15,546
Add: Other comprehensive income (loss) attributable to noncontrolling interests									26	0	(43)
Add: Net income attributable to noncontrolling interests	$ 22	$ 24	$ 0	$ (6)	$ 16	$ 15	$ 10	$ 25	40	66	35
Total comprehensive income									15,373	20,319	15,538
Consolidating adjustments
Revenues
Dividends from subsidiaries									(2,355)	(23,347)	(22,854)
Interest revenue									0	0	0
Interest revenue—intercompany									0	0	0
Interest expense									0	0	0
Interest expense—intercompany									0	0	0
Net interest revenue									0	0	0
Commissions and fees									0	0	0
Commissions and fees—intercompany									0	0	0
Principal transactions									0	0	0
Principal transactions—intercompany									0	0	0
Other revenue									0	0	0
Other revenue—intercompany									0	0	0
Total non-interest revenues									0	0	0
Total revenues, net of interest expense									(2,355)	(23,347)	(22,854)
Provisions for credit losses and for benefits and claims									0	0	0
Operating expenses
Compensation and benefits									0	0	0
Compensation and benefits—intercompany									0	0	0
Other operating									0	0	0
Other operating—intercompany									0	0	0
Total operating expenses									0	0	0
Equity in undistributed income of subsidiaries									(9,894)	3,620	2,163
Income from continuing operations before income taxes									(12,249)	(19,727)	(20,691)
Provision for income taxes									0	0	0
Income from continuing operations									(12,249)	(19,727)	(20,691)
Income (loss) from discontinued operations, net of taxes									0	0	0
Net income before attribution of noncontrolling interests									(12,249)	(19,727)	(20,691)
Noncontrolling interests									0	0	0
Citigroup’s net income									(12,249)	(19,727)	(20,691)
Comprehensive income
Add: Other comprehensive income (loss)									(4,021)	(949)	(3,757)
Citigroup’s total comprehensive income									(16,270)	(20,676)	(24,448)
Add: Other comprehensive income (loss) attributable to noncontrolling interests									0	0	0
Add: Net income attributable to noncontrolling interests									0	0	0
Total comprehensive income									(16,270)	(20,676)	(24,448)
Citigroup parent company | Reportable legal entities
Revenues
Dividends from subsidiaries									2,355	23,347	22,854
Interest revenue									0	0	67
Interest revenue—intercompany									4,162	5,091	4,933
Interest expense									4,992	4,949	4,783
Interest expense—intercompany									502	1,038	1,198
Net interest revenue									(1,332)	(896)	(981)
Commissions and fees									0	0	0
Commissions and fees—intercompany									(36)	(21)	(2)
Principal transactions									(1,254)	(2,537)	(1,310)
Principal transactions—intercompany									693	1,252	(929)
Other revenue									(127)	767	1,373
Other revenue—intercompany									111	(55)	(107)
Total non-interest revenues									(613)	(594)	(975)
Total revenues, net of interest expense									410	21,857	20,898
Provisions for credit losses and for benefits and claims									0	0	0
Operating expenses
Compensation and benefits									(5)	32	4
Compensation and benefits—intercompany									191	134	115
Other operating									37	(16)	(192)
Other operating—intercompany									15	20	49
Total operating expenses									238	170	(24)
Equity in undistributed income of subsidiaries									9,894	(3,620)	(2,163)
Income from continuing operations before income taxes									10,066	18,067	18,759
Provision for income taxes									(981)	(1,334)	714
Income from continuing operations									11,047	19,401	18,045
Income (loss) from discontinued operations, net of taxes									0	0	0
Net income before attribution of noncontrolling interests									11,047	19,401	18,045
Noncontrolling interests									0	0	0
Citigroup’s net income									11,047	19,401	18,045
Comprehensive income
Add: Other comprehensive income (loss)									4,260	852	(2,499)
Citigroup’s total comprehensive income									15,307	20,253	15,546
Add: Other comprehensive income (loss) attributable to noncontrolling interests									0	0	0
Add: Net income attributable to noncontrolling interests									0	0	0
Total comprehensive income									15,307	20,253	15,546
CGMHI | Reportable legal entities
Revenues
Dividends from subsidiaries									0	0	0
Interest revenue									5,364	10,661	8,732
Interest revenue—intercompany									920	1,942	1,659
Interest expense									1,989	7,010	5,430
Interest expense—intercompany									2,170	4,243	3,539
Net interest revenue									2,125	1,350	1,422
Commissions and fees									6,216	5,265	5,146
Commissions and fees—intercompany									290	354	237
Principal transactions									(4,252)	277	1,599
Principal transactions—intercompany									9,064	2,464	1,328
Other revenue									706	832	710
Other revenue—intercompany									23	102	143
Total non-interest revenues									12,047	9,294	9,163
Total revenues, net of interest expense									14,172	10,644	10,585
Provisions for credit losses and for benefits and claims									(1)	0	(22)
Operating expenses
Compensation and benefits									4,941	4,680	4,484
Compensation and benefits—intercompany									0	0	0
Other operating									2,393	2,326	2,224
Other operating—intercompany									2,317	2,410	2,312
Total operating expenses									9,651	9,416	9,020
Equity in undistributed income of subsidiaries									0	0	0
Income from continuing operations before income taxes									4,522	1,228	1,587
Provision for income taxes									1,249	176	1,123
Income from continuing operations									3,273	1,052	464
Income (loss) from discontinued operations, net of taxes									0	0	0
Net income before attribution of noncontrolling interests									3,273	1,052	464
Noncontrolling interests									0	0	0
Citigroup’s net income									3,273	1,052	464
Comprehensive income
Add: Other comprehensive income (loss)									(223)	(651)	257
Citigroup’s total comprehensive income									3,050	401	721
Add: Other comprehensive income (loss) attributable to noncontrolling interests									0	0	0
Add: Net income attributable to noncontrolling interests									0	0	0
Total comprehensive income									3,050	401	721
Other Citigroup subsidiaries and eliminations | Reportable legal entities
Revenues
Dividends from subsidiaries									0	0	0
Interest revenue									52,725	65,849	62,029
Interest revenue—intercompany									(5,082)	(7,033)	(6,592)
Interest expense									7,560	17,204	14,053
Interest expense—intercompany									(2,672)	(5,281)	(4,737)
Net interest revenue									42,755	46,893	46,121
Commissions and fees									5,169	6,481	6,711
Commissions and fees—intercompany									(254)	(333)	(235)
Principal transactions									19,391	11,152	8,616
Principal transactions—intercompany									(9,757)	(3,716)	(399)
Other revenue									4,901	4,702	3,447
Other revenue—intercompany									(134)	(47)	(36)
Total non-interest revenues									19,316	18,239	18,104
Total revenues, net of interest expense									62,071	65,132	64,225
Provisions for credit losses and for benefits and claims									17,496	8,383	7,590
Operating expenses
Compensation and benefits									17,278	16,721	16,666
Compensation and benefits—intercompany									(191)	(134)	(115)
Other operating									18,527	18,259	18,655
Other operating—intercompany									(2,332)	(2,430)	(2,361)
Total operating expenses									33,282	32,416	32,845
Equity in undistributed income of subsidiaries									0	0	0
Income from continuing operations before income taxes									11,293	24,333	23,790
Provision for income taxes									2,257	5,588	3,520
Income from continuing operations									9,036	18,745	20,270
Income (loss) from discontinued operations, net of taxes									(20)	(4)	(8)
Net income before attribution of noncontrolling interests									9,016	18,741	20,262
Noncontrolling interests									40	66	35
Citigroup’s net income									8,976	18,675	20,227
Comprehensive income
Add: Other comprehensive income (loss)									4,244	1,600	3,500
Citigroup’s total comprehensive income									13,220	20,275	23,727
Add: Other comprehensive income (loss) attributable to noncontrolling interests									26	0	(43)
Add: Net income attributable to noncontrolling interests									40	66	35
Total comprehensive income									$ 13,286	$ 20,341	$ 23,719