SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Variable Interest Entity
Total involvement with SPE assets	$ 463,641	$ 475,664
Significant unconsolidated VIE assets	403,959	409,476
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	57,196	58,909
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,455	3,691
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	15,977	16,883
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	175	136
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	76,803	79,619
Assets	2,405,686	2,411,834
Consolidated VIEs
Funded and Unfunded Exposure
Assets	59,682	66,188
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	6,000	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	6,600	8,500
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	29,915	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
Funded and Unfunded Exposure
Assets	29,915	31,852
Mortgage securitizations - U.S. agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	113,959	123,787
Significant unconsolidated VIE assets	113,959	123,787
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,501	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	130	136
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,631	2,468
Mortgage securitizations - U.S. agency-sponsored | Consolidated VIEs
Funded and Unfunded Exposure
Assets	0	0
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	59,901	64,963
Significant unconsolidated VIE assets	59,901	64,963
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3,299	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	157	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	3,456	3,880
Mortgage securitizations - Non-agency-sponsored | Consolidated VIEs
Funded and Unfunded Exposure
Assets	0	0
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	20,413	21,097
Significant unconsolidated VIE assets	320	0
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3	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	34	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	37	0
Citi-administered asset-backed commercial paper conduits | Consolidated VIEs
Funded and Unfunded Exposure
Assets	20,093	21,097
Collateralized loan obligations (CLOs)
Variable Interest Entity
Total involvement with SPE assets	4,932	5,562
Significant unconsolidated VIE assets	4,932	5,562
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1,924	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	1,924	2,344
Collateralized loan obligations (CLOs) | Consolidated VIEs
Funded and Unfunded Exposure
Assets	0	0
Asset-backed securities
Variable Interest Entity
Total involvement with SPE assets	211,948	204,680
Significant unconsolidated VIE assets	203,446	192,483
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	47,104	48,187
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	831	902
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	13,233	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	61,168	62,744
Asset-backed securities | Consolidated VIEs
Funded and Unfunded Exposure
Assets	8,502	12,197
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	1,025	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
Funded and Unfunded Exposure
Assets	1,025	883
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	20,528	21,317
Significant unconsolidated VIE assets	20,525	21,314
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,333	2,243
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	2,611	2,779
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,459	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	7,403	7,609
Municipal investments | Consolidated VIEs
Funded and Unfunded Exposure
Assets	3	3
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	388	368
Significant unconsolidated VIE assets	309	282
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	28	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	45	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	73	37
Client intermediation | Consolidated VIEs
Funded and Unfunded Exposure
Assets	79	86
Investment funds
Variable Interest Entity
Total involvement with SPE assets	632	545
Significant unconsolidated VIE assets	567	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	13	10
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	94	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	111	108
Investment funds | Consolidated VIEs
Funded and Unfunded Exposure
Assets	$ 65	$ 70