SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entity
Total involvement with SPE assets	$ 485,411	$ 460,157
Consolidated VIE / SPE assets	60,056	69,506
Significant unconsolidated VIE assets	425,355	390,651
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	36,581	34,320
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	5,643	5,425
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	13,783	16,157
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	118	74
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	56,125	55,976
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	78,000	69,000
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	425	711
Venture capital funds
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	57,000	52,500
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	32,420	43,534
Consolidated VIE / SPE assets	32,420	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	123,999	117,374
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	123,999	117,374
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1,948	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	61	72
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,009	2,743
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	46,132	39,608
Consolidated VIE / SPE assets	939	1,187
Significant unconsolidated VIE assets	45,193	38,421
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,550	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	2	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	2,553	877
Citi-administered asset-backed commercial paper conduits (ABCP)
Variable Interest Entity
Total involvement with SPE assets	16,730	15,622
Consolidated VIE / SPE assets	16,730	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	18,332	17,395
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	18,332	17,395
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4,273	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	4,273	4,199
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	222,274	196,728
Consolidated VIE / SPE assets	8,069	6,139
Significant unconsolidated VIE assets	214,205	190,589
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	25,153	23,756
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	1,587	1,151
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	9,114	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	35,854	34,431
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	3,349	6,950
Consolidated VIE / SPE assets	835	1,458
Significant unconsolidated VIE assets	2,514	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	1,611	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	1,611	3,548
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	20,335	20,312
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	20,335	20,312
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,569	2,636
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,056	4,274
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,041	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	9,666	9,944
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	1,352	1,455
Consolidated VIE / SPE assets	910	1,391
Significant unconsolidated VIE assets	442	64
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	88	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	56	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	144	4
Investment funds
Variable Interest Entity
Total involvement with SPE assets	488	827
Consolidated VIE / SPE assets	153	174
Significant unconsolidated VIE assets	335	653
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	0	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	0	1
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	15	22
Other
Variable Interest Entity
Total involvement with SPE assets	0	352
Consolidated VIE / SPE assets	0	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	$ 5,220	$ 6,000