SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Variable Interest Entity
Total involvement with SPE assets	$ 565,199	$ 528,659
Consolidated VIE/SPE assets	2,571,514	2,352,945
Significant unconsolidated VIE assets	509,069	468,620
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	65,892	64,618
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,411	3,483
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	17,372	16,351
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	172	172
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	86,847	84,624
Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	56,130	60,039
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	82,600	45,500
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	875	824
Venture capital investments
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	8,400	8,100
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	27,809	29,746
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
Credit card securitizations | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	27,809	29,746
Mortgage securitizations - U.S. agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	122,108	120,568
Significant unconsolidated VIE assets	122,108	120,568
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3,264	2,387
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	121	123
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	3,385	2,510
Mortgage securitizations - U.S. agency-sponsored | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	0	0
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	63,748	62,378
Significant unconsolidated VIE assets	63,748	62,378
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3,706	3,479
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	323	566
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	4,029	4,045
Mortgage securitizations - Non-agency-sponsored | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	0	0
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	18,285	21,306
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
Citi-administered asset-backed commercial paper conduits | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	18,285	21,306
Collateralized loan obligations (CLOs)
Variable Interest Entity
Total involvement with SPE assets	2,592	3,920
Significant unconsolidated VIE assets	2,592	3,920
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1,077	2,019
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	1,077	2,019
Collateralized loan obligations (CLOs) | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	0	0
Asset-backed securities
Variable Interest Entity
Total involvement with SPE assets	307,174	268,498
Significant unconsolidated VIE assets	298,749	260,551
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	55,400	54,349
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	706	735
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	13,647	13,185
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	69,753	68,269
Asset-backed securities | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	8,425	7,947
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	1,516	935
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
Municipal securities tender option bond trusts (TOBs) | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	1,516	935
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	20,740	20,280
Significant unconsolidated VIE assets	20,737	20,277
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,430	2,360
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	2,671	2,730
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,297	2,502
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	8,398	7,592
Municipal investments | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	3	3
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	377	387
Significant unconsolidated VIE assets	299	306
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	11	20
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	49
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	62	69
Client intermediation | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	78	81
Investment funds
Variable Interest Entity
Total involvement with SPE assets	850	641
Significant unconsolidated VIE assets	836	620
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4	4
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	34	18
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	105	98
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	143	120
Investment funds | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	$ 14	$ 21