CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statements of Income and Comprehensive Income (Details) - USD ($) $ 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
Revenues
Dividends from subsidiaries										$ 0	$ 0	$ 0
Interest revenue										76,510	70,828	61,579
Interest revenue—intercompany										0	0	0
Interest expense										29,163	24,266	16,518
Interest expense—intercompany										0	0	0
Net interest revenue										47,347	46,562	45,061
Commissions and fees										11,746	11,857	12,707
Commissions and fees—intercompany										0	0	0
Principal transactions										8,892	8,905	8,940
Principal transactions—intercompany										0	0	0
Other income										6,301	5,530	5,736
Other income—intercompany										0	0	0
Total non-interest revenues										26,939	26,292	27,383
Total 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
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
Operating expenses
Compensation and benefits										21,433	21,154	21,181
Compensation and benefits—intercompany										0	0	0
Other operating										20,569	20,687	21,051
Other operating—intercompany										0	0	0
Total operating expenses		10,454	10,464	10,500	10,584	9,893	10,311	10,712	10,925	42,002	41,841	42,232
Equity in undistributed income of subsidiaries										0	0	0
Income (loss) from continuing operations before income taxes										23,901	23,445	22,761
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)
Comprehensive income
Add: Other comprehensive income (loss)	[1]									852	(2,499)	(2,791)
Citigroup’s total comprehensive income (loss)										20,253	15,546	(9,589)
Add: Other comprehensive income (loss) attributable to noncontrolling interests										0	(43)	114
Noncontrolling interests		$ 16	$ 15	$ 10	$ 25	$ (16)	$ 3	$ 26	$ 22	66	35	60
Total comprehensive income (loss)										20,319	15,538	(9,415)
Consolidating adjustments
Revenues
Dividends from subsidiaries										(23,347)	(22,854)	(22,499)
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 income										0	0	0
Other income—intercompany										0	0	0
Total non-interest revenues										0	0	0
Total revenues, net of interest expense										(23,347)	(22,854)	(22,499)
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										3,620	2,163	19,088
Income (loss) from continuing operations before income taxes										(19,727)	(20,691)	(3,411)
Provision for income taxes										0	0	0
Income (loss) from continuing operations										(19,727)	(20,691)	(3,411)
Income (loss) from discontinued operations, net of taxes										0	0	0
Net income (loss) before attribution of noncontrolling interests										(19,727)	(20,691)	(3,411)
Noncontrolling interests										0	0	0
Citigroup’s net income (loss)										(19,727)	(20,691)	(3,411)
Comprehensive income
Add: Other comprehensive income (loss)										(949)	(3,757)	4,277
Citigroup’s total comprehensive income (loss)										(20,676)	(24,448)	866
Add: Other comprehensive income (loss) attributable to noncontrolling interests										0	0	0
Noncontrolling interests										0	0	0
Total comprehensive income (loss)										(20,676)	(24,448)	866
Citigroup parent company | Reportable legal entities
Revenues
Dividends from subsidiaries										23,347	22,854	22,499
Interest revenue										0	67	1
Interest revenue—intercompany										5,091	4,933	3,972
Interest expense										4,949	4,783	4,766
Interest expense—intercompany										1,038	1,198	829
Net interest revenue										(896)	(981)	(1,622)
Commissions and fees										0	0	0
Commissions and fees—intercompany										(21)	(2)	(2)
Principal transactions										(2,537)	(1,310)	1,654
Principal transactions—intercompany										1,252	(929)	934
Other income										767	1,373	(2,581)
Other income—intercompany										(55)	(107)	5
Total non-interest revenues										(594)	(975)	10
Total revenues, net of interest expense										21,857	20,898	20,887
Provisions for credit losses and for benefits and claims										0	0	0
Operating expenses
Compensation and benefits										32	4	(107)
Compensation and benefits—intercompany										134	115	120
Other operating										(16)	(192)	(318)
Other operating—intercompany										20	49	(35)
Total operating expenses										170	(24)	(340)
Equity in undistributed income of subsidiaries										(3,620)	(2,163)	(19,088)
Income (loss) from continuing operations before income taxes										18,067	18,759	2,139
Provision for income taxes										(1,334)	714	8,937
Income (loss) from continuing operations										19,401	18,045	(6,798)
Income (loss) from discontinued operations, net of taxes										0	0	0
Net income (loss) before attribution of noncontrolling interests										19,401	18,045	(6,798)
Noncontrolling interests										0	0	0
Citigroup’s net income (loss)										19,401	18,045	(6,798)
Comprehensive income
Add: Other comprehensive income (loss)										852	(2,499)	(2,791)
Citigroup’s total comprehensive income (loss)										20,253	15,546	(9,589)
Add: Other comprehensive income (loss) attributable to noncontrolling interests										0	0	0
Noncontrolling interests										0	0	0
Total comprehensive income (loss)										20,253	15,546	(9,589)
CGMHI | Reportable legal entities
Revenues
Dividends from subsidiaries										0	0	0
Interest revenue										10,661	8,732	5,279
Interest revenue—intercompany										1,942	1,659	1,178
Interest expense										7,010	5,430	2,340
Interest expense—intercompany										4,243	3,539	2,297
Net interest revenue										1,350	1,422	1,820
Commissions and fees										5,265	5,146	5,366
Commissions and fees—intercompany										354	237	182
Principal transactions										277	1,599	1,183
Principal transactions—intercompany										2,464	1,328	1,200
Other income										832	710	867
Other income—intercompany										102	143	170
Total non-interest revenues										9,294	9,163	8,968
Total revenues, net of interest expense										10,644	10,585	10,788
Provisions for credit losses and for benefits and claims										0	(22)	0
Operating expenses
Compensation and benefits										4,680	4,484	4,403
Compensation and benefits—intercompany										0	0	0
Other operating										2,326	2,224	2,184
Other operating—intercompany										2,410	2,312	2,231
Total operating expenses										9,416	9,020	8,818
Equity in undistributed income of subsidiaries										0	0	0
Income (loss) from continuing operations before income taxes										1,228	1,587	1,970
Provision for income taxes										176	1,123	873
Income (loss) from continuing operations										1,052	464	1,097
Income (loss) from discontinued operations, net of taxes										0	0	0
Net income (loss) before attribution of noncontrolling interests										1,052	464	1,097
Noncontrolling interests										0	0	(1)
Citigroup’s net income (loss)										1,052	464	1,098
Comprehensive income
Add: Other comprehensive income (loss)										(651)	257	(117)
Citigroup’s total comprehensive income (loss)										401	721	981
Add: Other comprehensive income (loss) attributable to noncontrolling interests										0	0	0
Noncontrolling interests										0	0	(1)
Total comprehensive income (loss)										401	721	980
Other Citigroup subsidiaries and eliminations | Reportable legal entities
Revenues
Dividends from subsidiaries										0	0	0
Interest revenue										65,849	62,029	56,299
Interest revenue—intercompany										(7,033)	(6,592)	(5,150)
Interest expense										17,204	14,053	9,412
Interest expense—intercompany										(5,281)	(4,737)	(3,126)
Net interest revenue										46,893	46,121	44,863
Commissions and fees										6,481	6,711	7,341
Commissions and fees—intercompany										(333)	(235)	(180)
Principal transactions										11,152	8,616	6,103
Principal transactions—intercompany										(3,716)	(399)	(2,134)
Other income										4,702	3,447	7,450
Other income—intercompany										(47)	(36)	(175)
Total non-interest revenues										18,239	18,104	18,405
Total revenues, net of interest expense										65,132	64,225	63,268
Provisions for credit losses and for benefits and claims										8,383	7,590	7,451
Operating expenses
Compensation and benefits										16,721	16,666	16,885
Compensation and benefits—intercompany										(134)	(115)	(120)
Other operating										18,259	18,655	19,185
Other operating—intercompany										(2,430)	(2,361)	(2,196)
Total operating expenses										32,416	32,845	33,754
Equity in undistributed income of subsidiaries										0	0	0
Income (loss) from continuing operations before income taxes										24,333	23,790	22,063
Provision for income taxes										5,588	3,520	19,578
Income (loss) from continuing operations										18,745	20,270	2,485
Income (loss) from discontinued operations, net of taxes										(4)	(8)	(111)
Net income (loss) before attribution of noncontrolling interests										18,741	20,262	2,374
Noncontrolling interests										66	35	61
Citigroup’s net income (loss)										18,675	20,227	2,313
Comprehensive income
Add: Other comprehensive income (loss)										1,600	3,500	(4,160)
Citigroup’s total comprehensive income (loss)										20,275	23,727	(1,847)
Add: Other comprehensive income (loss) attributable to noncontrolling interests										0	(43)	114
Noncontrolling interests										66	35	61
Total comprehensive income (loss)										$ 20,341	$ 23,719	$ (1,672)

[1] Includes the impact of ASU 2018-02, adopted in 2017. See Note 1 to the Consolidated Financial Statements.