SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Variable Interest Entity
Total involvement with SPE assets	$ 479,546	$ 475,664
Consolidated VIE/SPE assets	2,430,663	2,411,834
Significant unconsolidated VIE assets	421,984	409,476
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	58,426	58,909
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,480	3,691
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	16,476	16,883
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	180	136
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	78,562	79,619
Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	57,562	66,188
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	7,000	6,000
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	245	282
Venture capital investments
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	7,300	8,500
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	29,148	31,852
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	29,148	31,852
Mortgage securitizations - U.S. agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	121,689	123,787
Significant unconsolidated VIE assets	121,689	123,787
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,971	2,332
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	126	136
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	3,097	2,468
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	66,343	64,963
Significant unconsolidated VIE assets	66,343	64,963
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3,491	3,751
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	229	129
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	3,720	3,880
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	20,631	21,097
Significant unconsolidated VIE assets	312	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	38	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	38	0
Citi-administered asset-backed commercial paper conduits | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	20,319	21,097
Collateralized loan obligations (CLOs)
Variable Interest Entity
Total involvement with SPE assets	3,064	5,562
Significant unconsolidated VIE assets	3,064	5,562
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1,157	2,344
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	1,157	2,344
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	216,190	204,680
Significant unconsolidated VIE assets	209,234	192,483
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	48,406	48,187
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	783	902
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	13,642	13,655
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	62,831	62,744
Asset-backed securities | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	6,956	12,197
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	989	1,493
Significant unconsolidated VIE assets	0	610
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	0	12
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	417
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	429
Municipal securities tender option bond trusts (TOBs) | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	989	883
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	20,374	21,317
Significant unconsolidated VIE assets	20,371	21,314
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,377	2,243
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	2,682	2,779
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,465	2,587
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	7,524	7,609
Municipal investments | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	3	3
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	392	368
Significant unconsolidated VIE assets	310	282
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	20	37
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	54	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	74	37
Client intermediation | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	82	86
Investment funds
Variable Interest Entity
Total involvement with SPE assets	726	545
Significant unconsolidated VIE assets	661	475
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4	3
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	15	10
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	102	95
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	121	108
Investment funds | Consolidated VIEs
Variable Interest Entity
Consolidated VIE/SPE assets	$ 65	$ 70