SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entity
Total involvement with SPE assets	$ 312,836	$ 319,569
Consolidated VIE/SPE assets	67,978	76,394
Significant unconsolidated VIE assets	244,858	243,175
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	31,687	31,331
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,407	4,239
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	15,613	11,304
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	138	140
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	51,845	47,014
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	45,319	50,795
Consolidated VIE/SPE assets	45,319	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	113,565	116,610
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	113,565	116,610
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,965	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	68	74
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	3,033	2,721
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	25,452	22,251
Consolidated VIE/SPE assets	1,580	2,035
Significant unconsolidated VIE assets	23,872	20,216
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	356	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	357	331
Citi-administered asset-backed commercial paper conduits (ABCP)
Variable Interest Entity
Total involvement with SPE assets	17,435	19,282
Consolidated VIE/SPE assets	17,435	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	17,870	20,588
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	17,870	20,588
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	5,524	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	5,533	5,965
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	64,817	60,472
Consolidated VIE/SPE assets	639	633
Significant unconsolidated VIE assets	64,178	59,839
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	20,060	19,478
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	601	583
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	9,214	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	29,875	25,939
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	8,016	6,925
Consolidated VIE/SPE assets	2,029	2,166
Significant unconsolidated VIE assets	5,987	4,759
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	37	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	4,106	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	4,143	3,173
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	17,765	19,119
Consolidated VIE/SPE assets	1	7
Significant unconsolidated VIE assets	17,764	19,112
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,622	2,709
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,798	3,640
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,268	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	8,688	8,693
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	592	958
Consolidated VIE/SPE assets	419	824
Significant unconsolidated VIE assets	173	134
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	72	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	9	9
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	81	41
Investment funds
Variable Interest Entity
Total involvement with SPE assets	1,353	1,892
Consolidated VIE/SPE assets	525	616
Significant unconsolidated VIE assets	828	1,276
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	12	14
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	7
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3	13
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	5	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	20	34
Other
Variable Interest Entity
Total involvement with SPE assets	652	677
Consolidated VIE/SPE assets	31	36
Significant unconsolidated VIE assets	621	641
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	39	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	22	34
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	46	47
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	115	117
Mortgage-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 8,000	$ 9,000