SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Variable Interest Entity
Total involvement with SPE assets	$ 502,680	$ 500,212
Consolidated VIE/SPE assets	56,134	58,637
Significant unconsolidated VIE assets	446,546	441,575
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	46,246	41,528
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,458	4,756
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	17,245	17,266
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	115	268
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	68,064	63,818
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	86,000	100,000
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	499	497
Venture capital investments
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	50,500	55,600
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	31,266	31,518
Consolidated VIE/SPE assets	31,266	31,518
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	115,494	113,641
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	115,494	113,641
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,100	1,582
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	51	43
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,151	1,625
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	60,155	60,851
Consolidated VIE/SPE assets	0	632
Significant unconsolidated VIE assets	60,155	60,219
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,747	2,479
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	5	5
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	2,752	2,484
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	14,291	14,018
Consolidated VIE/SPE assets	14,291	14,018
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	8,259	8,302
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	8,259	8,302
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,599	2,636
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	2,599	2,636
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	246,990	246,632
Consolidated VIE/SPE assets	9,439	11,085
Significant unconsolidated VIE assets	237,551	235,547
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	36,051	32,242
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	1,088	1,139
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	11,639	12,189
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	48,778	45,570
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	2,839	3,251
Consolidated VIE/SPE assets	678	905
Significant unconsolidated VIE assets	2,161	2,346
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	8	2
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	1,571	1,498
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	1,579	1,500
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	22,191	20,597
Consolidated VIE/SPE assets	3	3
Significant unconsolidated VIE assets	22,188	20,594
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,673	2,512
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,368	3,617
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	4,010	3,562
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	10,051	9,691
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	810	904
Consolidated VIE/SPE assets	373	297
Significant unconsolidated VIE assets	437	607
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	66	75
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	63	224
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	129	299
Investment funds
Variable Interest Entity
Total involvement with SPE assets	385	498
Consolidated VIE/SPE assets	84	179
Significant unconsolidated VIE assets	301	319
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2	0
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	2	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	20	12
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	25	13
Other
Variable Interest Entity
Total involvement with SPE assets	0	0
Consolidated VIE/SPE assets	0	0
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