SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Variable Interest Entity
Total involvement with SPE assets	$ 468,355	$ 460,237
Consolidated VIE/SPE assets	58,605	69,506
Significant unconsolidated VIE assets	409,750	390,731
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	34,741	34,324
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	5,300	5,425
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	14,396	16,157
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	113	74
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	54,550	55,980
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	74,800	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	53,700	52,500
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	32,275	43,534
Consolidated VIE/SPE assets	32,275	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 securitizations - U.S. agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	119,320	117,374
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	119,320	117,374
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,078	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	75	72
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,153	2,743
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	41,951	39,608
Consolidated VIE/SPE assets	934	1,187
Significant unconsolidated VIE assets	41,017	38,421
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1,194	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	1,195	877
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	16,457	15,622
Consolidated VIE/SPE assets	16,457	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	19,071	17,395
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	19,071	17,395
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4,331	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	4,331	4,199
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	213,880	196,728
Consolidated VIE/SPE assets	7,311	6,139
Significant unconsolidated VIE assets	206,569	190,589
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	24,620	23,756
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	1,133	1,151
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	9,748	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	35,501	34,431
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	3,867	6,950
Consolidated VIE/SPE assets	843	1,458
Significant unconsolidated VIE assets	3,024	5,492
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	0	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	1,869	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	1,869	3,548
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	19,987	20,312
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	19,987	20,312
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,429	2,636
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,167	4,274
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,763	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	9,359	9,944
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	1,074	1,535
Consolidated VIE/SPE assets	652	1,391
Significant unconsolidated VIE assets	422	144
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	88	8
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	36	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	124	8
Investment funds
Variable Interest Entity
Total involvement with SPE assets	472	827
Consolidated VIE/SPE assets	132	174
Significant unconsolidated VIE assets	340	653
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1	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	16	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	1	352
Consolidated VIE/SPE assets	1	1
Significant unconsolidated VIE assets	0	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	0	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	0	208
Mortgage-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 5,400	$ 6,000