SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Variable Interest Entity
Total involvement with SPE assets	$ 308,003	$ 319,569
Consolidated VIE/SPE assets	72,116	76,394
Significant unconsolidated VIE assets	235,887	243,175
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	30,439	31,331
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,218	4,239
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	12,423	11,304
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	153	140
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	47,233	47,014
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	46,540	50,795
Consolidated VIE/SPE assets	46,540	50,795
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	111,980	116,610
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	111,980	116,610
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,690	2,647
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	87	74
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,777	2,721
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	20,660	22,251
Consolidated VIE/SPE assets	1,891	2,035
Significant unconsolidated VIE assets	18,769	20,216
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	335	330
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	1	1
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	336	331
Citi-administered asset-backed commercial paper conduits (ABCP)
Variable Interest Entity
Total involvement with SPE assets	18,962	19,282
Consolidated VIE/SPE assets	18,962	19,282
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	16,491	20,588
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	16,491	20,588
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	5,362	5,956
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	9
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	5,371	5,965
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	63,019	60,472
Consolidated VIE/SPE assets	637	633
Significant unconsolidated VIE assets	62,382	59,839
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	19,190	19,478
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	571	583
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	6,904	5,878
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	26,665	25,939
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	7,105	6,925
Consolidated VIE/SPE assets	2,165	2,166
Significant unconsolidated VIE assets	4,940	4,759
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	18	138
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,344	3,035
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	3,362	3,173
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	19,265	19,119
Consolidated VIE/SPE assets	5	7
Significant unconsolidated VIE assets	19,260	19,112
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,769	2,709
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,632	3,640
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,129	2,344
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	8,530	8,693
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	1,414	958
Consolidated VIE/SPE assets	1,279	824
Significant unconsolidated VIE assets	135	134
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	37	32
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	7	9
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	44	41
Investment funds
Variable Interest Entity
Total involvement with SPE assets	1,874	1,892
Consolidated VIE/SPE assets	603	616
Significant unconsolidated VIE assets	1,271	1,276
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	10	14
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	7	7
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	13	13
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	30	34
Other
Variable Interest Entity
Total involvement with SPE assets	693	677
Consolidated VIE/SPE assets	34	36
Significant unconsolidated VIE assets	659	641
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	28	27
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	8	9
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	33	34
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	49	47
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	118	117
Mortgage-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 8,000	$ 9,000