SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Variable Interest Entity
Total involvement with SPE assets	$ 462,731	$ 460,157
Consolidated VIE/SPE assets	60,424	69,506
Significant unconsolidated VIE assets	402,307	390,651
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	36,305	34,320
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	5,368	5,425
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	15,593	16,157
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	56	74
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	57,322	55,976
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	70,400	69,000
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	710	711
Venture capital investments
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	52,400	52,500
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	33,838	43,534
Consolidated VIE/SPE assets	33,838	43,534
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. government-sponsored agency guaranteed
Variable Interest Entity
Total involvement with SPE assets	115,290	117,374
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	115,290	117,374
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,103	2,671
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	54	72
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,157	2,743
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	43,493	39,608
Consolidated VIE/SPE assets	982	1,187
Significant unconsolidated VIE assets	42,511	38,421
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1,043	876
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	1,044	877
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	16,028	15,622
Consolidated VIE/SPE assets	16,028	15,622
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	17,986	17,395
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	17,986	17,395
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4,272	4,199
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	4,272	4,199
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	209,806	196,728
Consolidated VIE/SPE assets	7,660	6,139
Significant unconsolidated VIE assets	202,146	190,589
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	26,129	23,756
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	1,131	1,151
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	10,302	9,524
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	37,562	34,431
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	4,747	6,950
Consolidated VIE/SPE assets	1,113	1,458
Significant unconsolidated VIE assets	3,634	5,492
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	16	4
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,320	3,544
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	2,336	3,548
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	20,235	20,312
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	20,235	20,312
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,736	2,636
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,237	4,274
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,906	3,034
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	9,879	9,944
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	742	1,455
Consolidated VIE/SPE assets	676	1,391
Significant unconsolidated VIE assets	66	64
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4	4
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	4	4
Investment funds
Variable Interest Entity
Total involvement with SPE assets	515	827
Consolidated VIE/SPE assets	126	174
Significant unconsolidated VIE assets	389	653
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2	5
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	15	16
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	18	22
Other
Variable Interest Entity
Total involvement with SPE assets	51	352
Consolidated VIE/SPE assets	1	1
Significant unconsolidated VIE assets	50	351
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	0	169
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	50	39
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	50	208
Mortgage-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 6,000	$ 6,000