CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statements of Income and Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues
Dividends from subsidiaries	$ 0	$ 0	$ 0	$ 0
Interest revenue	15,821	14,653	45,445	43,176
Interest revenue—intercompany	0	0	0	0
Interest expense	4,379	3,174	11,981	9,234
Interest expense—intercompany	0	0	0	0
Net interest revenue	11,442	11,479	33,464	33,942
Commissions and fees	2,931	2,644	8,627	7,832
Commissions and fees—intercompany	0	0	0	0
Principal transactions	2,170	2,238	7,754	5,894
Principal transactions—intercompany	0	0	0	0
Other income	1,630	1,399	4,349	5,195
Other income—intercompany	0	0	0	0
Total non-interest revenues	6,731	6,281	20,730	18,921
Total revenues, net of interest expense	18,173	17,760	54,194	52,863
Provisions for credit losses and for benefits and claims	1,999	1,736	5,378	5,190
Operating expenses
Compensation and benefits	5,304	5,203	16,301	15,988
Compensation and benefits—intercompany	0	0	0	0
Other operating	4,867	5,201	14,853	15,308
Other operating—intercompany	0	0	0	0
Total operating expenses	10,171	10,404	31,154	31,296
Equity in undistributed income of subsidiaries	0	0	0	0
Income (loss) from continuing operations before income taxes	6,003	5,620	17,662	16,377
Provision for income taxes	1,866	1,733	5,524	4,935
Income from continuing operations	4,137	3,887	12,138	11,442
Income (loss) from discontinued operations, net of taxes	(5)	(30)	(2)	(55)
Net income before attribution of noncontrolling interests	4,132	3,857	12,136	11,387
Noncontrolling interests	(1)	17	41	48
Citigroup’s net income	4,133	3,840	12,095	11,339
Comprehensive income
Add: Other comprehensive income (loss)	8	(1,078)	1,986	2,166
Citigroup’s total comprehensive income	4,141	2,762	14,081	13,505
Add: Other comprehensive income attributable to noncontrolling interests	12	10	82	(13)
Add: Net income attributable to noncontrolling interests	(1)	17	41	48
Total comprehensive income (loss)	4,152	2,789	14,204	13,540
Reportable legal entities | Citigroup parent company
Revenues
Dividends from subsidiaries	5,360	4,000	11,625	9,700
Interest revenue	0	2	0	5
Interest revenue—intercompany	1,040	695	2,909	2,235
Interest expense	1,195	1,102	3,549	3,266
Interest expense—intercompany	240	61	593	140
Net interest revenue	(395)	(466)	(1,233)	(1,166)
Commissions and fees	0	0	0	0
Commissions and fees—intercompany	0	0	(1)	(19)
Principal transactions	610	(1,103)	1,569	(1,498)
Principal transactions—intercompany	168	977	768	1,018
Other income	(860)	482	(2,500)	(3,197)
Other income—intercompany	33	(501)	71	3,495
Total non-interest revenues	(49)	(145)	(93)	(201)
Total revenues, net of interest expense	4,916	3,389	10,299	8,333
Provisions for credit losses and for benefits and claims	0	0	0	0
Operating expenses
Compensation and benefits	(3)	26	(18)	18
Compensation and benefits—intercompany	46	8	97	34
Other operating	(17)	(103)	(333)	377
Other operating—intercompany	8	133	(41)	213
Total operating expenses	34	64	(295)	642
Equity in undistributed income of subsidiaries	(1,015)	120	755	2,773
Income (loss) from continuing operations before income taxes	3,867	3,445	11,349	10,464
Provision for income taxes	(266)	(395)	(746)	(875)
Income from continuing operations	4,133	3,840	12,095	11,339
Income (loss) from discontinued operations, net of taxes	0	0	0	0
Net income before attribution of noncontrolling interests	4,133	3,840	12,095	11,339
Noncontrolling interests	0	0	0	0
Citigroup’s net income	4,133	3,840	12,095	11,339
Comprehensive income
Add: Other comprehensive income (loss)	8	(1,078)	1,986	2,166
Citigroup’s total comprehensive income	4,141	2,762	14,081	13,505
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)	4,141	2,762	14,081	13,505
Reportable legal entities | CGMHI
Revenues
Dividends from subsidiaries	0	0	0	0
Interest revenue	1,439	1,158	3,870	3,555
Interest revenue—intercompany	313	148	847	423
Interest expense	642	345	1,584	1,110
Interest expense—intercompany	581	401	1,660	1,246
Net interest revenue	529	560	1,473	1,622
Commissions and fees	1,284	1,062	3,818	3,141
Commissions and fees—intercompany	13	63	123	33
Principal transactions	688	1,600	2,692	3,857
Principal transactions—intercompany	(249)	(470)	(641)	(1,513)
Other income	649	51	810	178
Other income—intercompany	(21)	51	6	250
Total non-interest revenues	2,364	2,357	6,808	5,946
Total revenues, net of interest expense	2,893	2,917	8,281	7,568
Provisions for credit losses and for benefits and claims	(1)	0	0	0
Operating expenses
Compensation and benefits	1,104	1,150	3,578	3,641
Compensation and benefits—intercompany	0	0	0	0
Other operating	457	444	1,306	1,242
Other operating—intercompany	517	379	1,487	1,008
Total operating expenses	2,078	1,973	6,371	5,891
Equity in undistributed income of subsidiaries	0	0	0	0
Income (loss) from continuing operations before income taxes	816	944	1,910	1,677
Provision for income taxes	324	345	800	539
Income from continuing operations	492	599	1,110	1,138
Income (loss) from discontinued operations, net of taxes	0	0	0	0
Net income before attribution of noncontrolling interests	492	599	1,110	1,138
Noncontrolling interests	0	(9)	0	(10)
Citigroup’s net income	492	608	1,110	1,148
Comprehensive income
Add: Other comprehensive income (loss)	(84)	(133)	(142)	(28)
Citigroup’s total comprehensive income	408	475	968	1,120
Add: Other comprehensive income attributable to noncontrolling interests	0	0	0	0
Add: Net income attributable to noncontrolling interests	0	(9)	0	(10)
Total comprehensive income (loss)	408	466	968	1,110
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Revenues
Dividends from subsidiaries	0	0	0	0
Interest revenue	14,382	13,493	41,575	39,616
Interest revenue—intercompany	(1,353)	(843)	(3,756)	(2,658)
Interest expense	2,542	1,727	6,848	4,858
Interest expense—intercompany	(821)	(462)	(2,253)	(1,386)
Net interest revenue	11,308	11,385	33,224	33,486
Commissions and fees	1,647	1,582	4,809	4,691
Commissions and fees—intercompany	(13)	(63)	(122)	(14)
Principal transactions	872	1,741	3,493	3,535
Principal transactions—intercompany	81	(507)	(127)	495
Other income	1,841	866	6,039	8,214
Other income—intercompany	(12)	450	(77)	(3,745)
Total non-interest revenues	4,416	4,069	14,015	13,176
Total revenues, net of interest expense	15,724	15,454	47,239	46,662
Provisions for credit losses and for benefits and claims	2,000	1,736	5,378	5,190
Operating expenses
Compensation and benefits	4,203	4,027	12,741	12,329
Compensation and benefits—intercompany	(46)	(8)	(97)	(34)
Other operating	4,427	4,860	13,880	13,689
Other operating—intercompany	(525)	(512)	(1,446)	(1,221)
Total operating expenses	8,059	8,367	25,078	24,763
Equity in undistributed income of subsidiaries	0	0	0	0
Income (loss) from continuing operations before income taxes	5,665	5,351	16,783	16,709
Provision for income taxes	1,808	1,783	5,470	5,271
Income from continuing operations	3,857	3,568	11,313	11,438
Income (loss) from discontinued operations, net of taxes	(5)	(30)	(2)	(55)
Net income before attribution of noncontrolling interests	3,852	3,538	11,311	11,383
Noncontrolling interests	(1)	26	41	58
Citigroup’s net income	3,853	3,512	11,270	11,325
Comprehensive income
Add: Other comprehensive income (loss)	(762)	(1,003)	(4,638)	171
Citigroup’s total comprehensive income	3,091	2,509	6,632	11,496
Add: Other comprehensive income attributable to noncontrolling interests	12	10	82	(13)
Add: Net income attributable to noncontrolling interests	(1)	26	41	58
Total comprehensive income (loss)	3,102	2,545	6,755	11,541
Consolidating adjustments
Revenues
Dividends from subsidiaries	(5,360)	(4,000)	(11,625)	(9,700)
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	(5,360)	(4,000)	(11,625)	(9,700)
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	1,015	(120)	(755)	(2,773)
Income (loss) from continuing operations before income taxes	(4,345)	(4,120)	(12,380)	(12,473)
Provision for income taxes	0	0	0	0
Income from continuing operations	(4,345)	(4,120)	(12,380)	(12,473)
Income (loss) from discontinued operations, net of taxes	0	0	0	0
Net income before attribution of noncontrolling interests	(4,345)	(4,120)	(12,380)	(12,473)
Noncontrolling interests	0	0	0	0
Citigroup’s net income	(4,345)	(4,120)	(12,380)	(12,473)
Comprehensive income
Add: Other comprehensive income (loss)	846	1,136	4,780	(143)
Citigroup’s total comprehensive income	(3,499)	(2,984)	(7,600)	(12,616)
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)	$ (3,499)	$ (2,984)	$ (7,600)	$ (12,616)