CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statements of Income and Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues
Dividends from subsidiaries	$ 0	$ 0	$ 0	$ 0
Interest revenue	18,170	15,914	52,052	45,729
Interest revenue—intercompany	0	0	0	0
Interest expense	6,368	4,379	17,413	11,981
Interest expense—intercompany	0	0	0	0
Net interest revenue	11,802	11,535	34,639	33,748
Commissions and fees	2,803	3,241	8,944	9,552
Commissions and fees—intercompany	0	0	0	0
Principal transactions	2,566	2,248	8,006	7,985
Principal transactions—intercompany	0	0	0	0
Other income	1,218	1,395	4,141	3,655
Other income—intercompany	0	0	0	0
Total non-interest revenues	6,587	6,884	21,091	21,192
Total revenues, net of interest expense	18,389	18,419	55,730	54,940
Provisions for credit losses and for benefits and claims	1,974	1,999	5,643	5,378
Operating expenses
Compensation and benefits	5,319	5,304	16,578	16,301
Compensation and benefits—intercompany	0	0	0	0
Other operating	4,992	5,113	15,370	15,599
Other operating—intercompany	0	0	0	0
Total operating expenses	10,311	10,417	31,948	31,900
Equity in undistributed income of subsidiaries	0	0	0	0
Income (loss) from continuing operations before income taxes	6,104	6,003	18,139	17,662
Provision for income taxes	1,471	1,866	4,356	5,524
Income from continuing operations	4,633	4,137	13,783	12,138
Loss from discontinued operations, net of taxes	(8)	(5)	0	(2)
Net income before attribution of noncontrolling interests	4,625	4,132	13,783	12,136
Noncontrolling interests	3	(1)	51	41
Citigroup’s net income	4,622	4,133	13,732	12,095
Comprehensive income
Add: Other comprehensive income (loss)	(1,151)	8	(3,974)	1,986
Citigroup’s total comprehensive income	3,471	4,141	9,758	14,081
Add: Other comprehensive income attributable to noncontrolling interests	8	12	(35)	82
Add: Net income attributable to noncontrolling interests	3	(1)	51	41
Total comprehensive income (loss)	3,482	4,152	9,774	14,204
Reportable legal entities | Citigroup parent company
Revenues
Dividends from subsidiaries	7,948	5,360	16,648	11,625
Interest revenue	1	0	67	0
Interest revenue—intercompany	1,281	1,040	3,636	2,909
Interest expense	1,068	1,195	3,119	3,549
Interest expense—intercompany	492	240	1,467	593
Net interest revenue	(278)	(395)	(883)	(1,233)
Commissions and fees	0	0	0	0
Commissions and fees—intercompany	0	0	(1)	(1)
Principal transactions	(100)	610	(275)	1,569
Principal transactions—intercompany	(303)	168	(1,161)	768
Other income	266	(860)	817	(2,500)
Other income—intercompany	(46)	32	(111)	70
Total non-interest revenues	(183)	(50)	(731)	(94)
Total revenues, net of interest expense	7,487	4,915	15,034	10,298
Provisions for credit losses and for benefits and claims	0	0	0	0
Operating expenses
Compensation and benefits	14	(3)	149	(18)
Compensation and benefits—intercompany	19	46	82	97
Other operating	(201)	(18)	(210)	(334)
Other operating—intercompany	13	8	38	(41)
Total operating expenses	(155)	33	59	(296)
Equity in undistributed income of subsidiaries	(3,098)	(1,015)	(2,060)	755
Income (loss) from continuing operations before income taxes	4,544	3,867	12,915	11,349
Provision for income taxes	(78)	(266)	(817)	(746)
Income from continuing operations	4,622	4,133	13,732	12,095
Loss from discontinued operations, net of taxes	0	0		0
Net income before attribution of noncontrolling interests	4,622	4,133	13,732	12,095
Noncontrolling interests	0	0	0	0
Citigroup’s net income	4,622	4,133	13,732	12,095
Comprehensive income
Add: Other comprehensive income (loss)	(1,151)	8	(3,974)	1,986
Citigroup’s total comprehensive income	3,471	4,141	9,758	14,081
Add: Other comprehensive income attributable to noncontrolling interests	0	0	0	0
Add: Net income attributable to noncontrolling interests	0	0	0	0
Total comprehensive income (loss)	3,471	4,141	9,758	14,081
Reportable legal entities | CGMHI
Revenues
Dividends from subsidiaries	0	0	0	0
Interest revenue	2,291	1,442	6,344	3,873
Interest revenue—intercompany	424	313	1,206	847
Interest expense	1,405	643	3,732	1,578
Interest expense—intercompany	899	580	2,567	1,666
Net interest revenue	411	532	1,251	1,476
Commissions and fees	1,194	1,262	3,793	3,933
Commissions and fees—intercompany	72	13	163	123
Principal transactions	581	501	805	2,377
Principal transactions—intercompany	(10)	(401)	1,461	(207)
Other income	325	729	666	868
Other income—intercompany	57	153	88	156
Total non-interest revenues	2,219	2,257	6,976	7,250
Total revenues, net of interest expense	2,630	2,789	8,227	8,726
Provisions for credit losses and for benefits and claims	3	(1)	(21)	0
Operating expenses
Compensation and benefits	1,148	1,104	3,695	3,578
Compensation and benefits—intercompany	0	0	0	0
Other operating	558	560	1,684	1,605
Other operating—intercompany	564	310	1,835	1,633
Total operating expenses	2,270	1,974	7,214	6,816
Equity in undistributed income of subsidiaries	0	0	0	0
Income (loss) from continuing operations before income taxes	357	816	1,034	1,910
Provision for income taxes	169	324	853	800
Income from continuing operations	188	492	181	1,110
Loss from discontinued operations, net of taxes	0	0		0
Net income before attribution of noncontrolling interests	188	492	181	1,110
Noncontrolling interests	0	0	0	0
Citigroup’s net income	188	492	181	1,110
Comprehensive income
Add: Other comprehensive income (loss)	(196)	(84)	(186)	(142)
Citigroup’s total comprehensive income	(8)	408	(5)	968
Add: Other comprehensive income attributable to noncontrolling interests	0	0	0	0
Add: Net income attributable to noncontrolling interests	0	0	0	0
Total comprehensive income (loss)	(8)	408	(5)	968
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Revenues
Dividends from subsidiaries	0	0	0	0
Interest revenue	15,878	14,472	45,641	41,856
Interest revenue—intercompany	(1,705)	(1,353)	(4,842)	(3,756)
Interest expense	3,895	2,541	10,562	6,854
Interest expense—intercompany	(1,391)	(820)	(4,034)	(2,259)
Net interest revenue	11,669	11,398	34,271	33,505
Commissions and fees	1,609	1,979	5,151	5,619
Commissions and fees—intercompany	(72)	(13)	(162)	(122)
Principal transactions	2,085	1,137	7,476	4,039
Principal transactions—intercompany	313	233	(300)	(561)
Other income	627	1,526	2,658	5,287
Other income—intercompany	(11)	(185)	23	(226)
Total non-interest revenues	4,551	4,677	14,846	14,036
Total revenues, net of interest expense	16,220	16,075	49,117	47,541
Provisions for credit losses and for benefits and claims	1,971	2,000	5,664	5,378
Operating expenses
Compensation and benefits	4,157	4,203	12,734	12,741
Compensation and benefits—intercompany	(19)	(46)	(82)	(97)
Other operating	4,635	4,571	13,896	14,328
Other operating—intercompany	(577)	(318)	(1,873)	(1,592)
Total operating expenses	8,196	8,410	24,675	25,380
Equity in undistributed income of subsidiaries	0	0	0	0
Income (loss) from continuing operations before income taxes	6,053	5,665	18,778	16,783
Provision for income taxes	1,380	1,808	4,320	5,470
Income from continuing operations	4,673	3,857	14,458	11,313
Loss from discontinued operations, net of taxes	(8)	(5)		(2)
Net income before attribution of noncontrolling interests	4,665	3,852	14,458	11,311
Noncontrolling interests	3	(1)	51	41
Citigroup’s net income	4,662	3,853	14,407	11,270
Comprehensive income
Add: Other comprehensive income (loss)	(458)	(762)	1,787	(4,638)
Citigroup’s total comprehensive income	4,204	3,091	16,194	6,632
Add: Other comprehensive income attributable to noncontrolling interests	8	12	(35)	82
Add: Net income attributable to noncontrolling interests	3	(1)	51	41
Total comprehensive income (loss)	4,215	3,102	16,210	6,755
Consolidating adjustments
Revenues
Dividends from subsidiaries	(7,948)	(5,360)	(16,648)	(11,625)
Interest revenue	0	0	0	0
Interest revenue—intercompany	0	0	0	0
Interest expense	0	0	0	0
Interest expense—intercompany	0	0	0	0
Net interest revenue	0	0	0	0
Commissions and fees	0	0	0	0
Commissions and fees—intercompany	0	0	0	0
Principal transactions	0	0	0	0
Principal transactions—intercompany	0	0	0	0
Other income	0	0	0	0
Other income—intercompany	0	0	0	0
Total non-interest revenues	0	0	0	0
Total revenues, net of interest expense	(7,948)	(5,360)	(16,648)	(11,625)
Provisions for credit losses and for benefits and claims	0	0	0	0
Operating expenses
Compensation and benefits	0	0	0	0
Compensation and benefits—intercompany	0	0	0	0
Other operating	0	0	0	0
Other operating—intercompany	0	0	0	0
Total operating expenses	0	0	0	0
Equity in undistributed income of subsidiaries	3,098	1,015	2,060	(755)
Income (loss) from continuing operations before income taxes	(4,850)	(4,345)	(14,588)	(12,380)
Provision for income taxes	0	0	0	0
Income from continuing operations	(4,850)	(4,345)	(14,588)	(12,380)
Loss from discontinued operations, net of taxes	0	0		0
Net income before attribution of noncontrolling interests	(4,850)	(4,345)	(14,588)	(12,380)
Noncontrolling interests	0	0	0	0
Citigroup’s net income	(4,850)	(4,345)	(14,588)	(12,380)
Comprehensive income
Add: Other comprehensive income (loss)	654	846	(1,601)	4,780
Citigroup’s total comprehensive income	(4,196)	(3,499)	(16,189)	(7,600)
Add: Other comprehensive income attributable to noncontrolling interests	0	0	0	0
Add: Net income attributable to noncontrolling interests	0	0	0	0
Total comprehensive income (loss)	$ (4,196)	$ (3,499)	$ (16,189)	$ (7,600)