SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Variable Interest Entity
Total involvement with SPE assets	$ 307,264	$ 401,822
Consolidated VIE / SPE assets	74,756	76,291
Significant unconsolidated VIE assets	232,508	325,531
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	27,493	28,523
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,507	4,219
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	10,968	10,422
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	128	190
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	43,096	43,354
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	49,739	50,171
Consolidated VIE / SPE assets	49,739	50,171
Significant unconsolidated VIE assets	0	0
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	0	0
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	0	0
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	0	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	0	0
Mortgage-backed securities - U.S. agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	116,257	214,458
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	116,257	214,458
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,528	3,852
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	0	0
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	63	78
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,591	3,930
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	21,123	15,965
Consolidated VIE / SPE assets	932	1,092
Significant unconsolidated VIE assets	20,191	14,873
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	280	312
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	36	35
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	0	0
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	1	1
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	317	348
Citi-administered asset-backed commercial paper conduits (ABCP)
Variable Interest Entity
Total involvement with SPE assets	19,298	19,693
Consolidated VIE / SPE assets	19,298	19,693
Significant unconsolidated VIE assets	0	0
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	0	0
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	0	0
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	0	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	0	0
Collateralized loan obligations (CLOs)
Variable Interest Entity
Total involvement with SPE assets	19,182	18,886
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	19,182	18,886
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	5,690	5,128
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	0	0
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	9	62
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	5,699	5,190
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	51,393	53,168
Consolidated VIE / SPE assets	672	733
Significant unconsolidated VIE assets	50,721	52,435
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	15,412	16,553
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	599	475
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	5,016	4,915
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	0	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	21,027	21,943
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	6,777	7,070
Consolidated VIE / SPE assets	2,178	2,843
Significant unconsolidated VIE assets	4,599	4,227
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	13	40
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,063	2,842
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	0	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	3,076	2,882
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	17,830	17,679
Consolidated VIE / SPE assets	11	14
Significant unconsolidated VIE assets	17,819	17,665
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,627	2,441
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,855	3,578
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,345	2,580
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	0	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	8,827	8,599
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	2,664	515
Consolidated VIE / SPE assets	1,131	371
Significant unconsolidated VIE assets	1,533	144
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	782	49
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	491	0
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	6	3
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	1,279	52
Investment funds
Variable Interest Entity
Total involvement with SPE assets	2,058	2,788
Consolidated VIE / SPE assets	762	767
Significant unconsolidated VIE assets	1,296	2,021
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	28	32
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	8	120
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	15	27
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	2	3
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	53	182
Other
Variable Interest Entity
Total involvement with SPE assets	943	1,429
Consolidated VIE / SPE assets	33	607
Significant unconsolidated VIE assets	910	822
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	133	116
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	9	11
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	38	58
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	47	43
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	227	228
Mortgage-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 9,000	$ 10,000