SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Variable Interest Entity
Total involvement with SPE assets	$ 457,679	$ 460,157
Consolidated VIE/SPE assets	66,136	69,506
Significant unconsolidated VIE assets	391,543	390,651
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	34,578	34,320
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	5,375	5,425
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	16,094	16,157
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	66	74
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	56,113	55,976
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	37,612	43,534
Consolidated VIE/SPE assets	37,612	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. agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	109,728	117,374
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	109,728	117,374
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,000	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	65	72
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,065	2,743
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	37,140	39,608
Consolidated VIE/SPE assets	941	1,187
Significant unconsolidated VIE assets	36,199	38,421
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1,054	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	1,055	877
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	19,386	15,622
Consolidated VIE/SPE assets	19,386	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	17,453	17,395
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	17,453	17,395
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3,918	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	3,918	4,199
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	207,817	196,728
Consolidated VIE/SPE assets	5,569	6,139
Significant unconsolidated VIE assets	202,248	190,589
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	24,935	23,756
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	1,154	1,151
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	9,863	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	35,952	34,431
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	6,129	6,950
Consolidated VIE/SPE assets	1,134	1,458
Significant unconsolidated VIE assets	4,995	5,492
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	3,125	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	3,129	3,548
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	20,383	20,312
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	20,383	20,312
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,662	2,636
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,221	4,274
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,030	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	9,913	9,944
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	1,430	1,455
Consolidated VIE/SPE assets	1,371	1,391
Significant unconsolidated VIE assets	59	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	539	827
Consolidated VIE/SPE assets	121	174
Significant unconsolidated VIE assets	418	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	0	1
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	17	22
Other
Variable Interest Entity
Total involvement with SPE assets	62	352
Consolidated VIE/SPE assets	2	1
Significant unconsolidated VIE assets	60	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	60	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	60	208
Mortgage-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 6,000	$ 6,000