SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Variable Interest Entity
Total involvement with SPE assets	$ 458,627	$ 475,664
Consolidated VIE/SPE assets	63,201	66,188
Significant unconsolidated VIE assets	395,426	409,476
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	56,729	58,909
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,562	3,691
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	16,455	16,883
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	146	136
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	76,892	79,619
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	281	282
Venture capital investments
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	8,100	8,500
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	29,948	31,852
Consolidated VIE/SPE assets	29,948	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
Mortgage securitizations - U.S. agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	109,256	123,787
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	109,256	123,787
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,493	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	133	136
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,626	2,468
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	59,255	64,963
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	59,255	64,963
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3,427	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	121	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	3,548	3,880
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	19,076	21,097
Consolidated VIE/SPE assets	19,076	21,097
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	5,343	5,562
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	5,343	5,562
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,205	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	2,205	2,344
Asset-backed securities
Variable Interest Entity
Total involvement with SPE assets	212,900	204,680
Consolidated VIE/SPE assets	13,166	12,197
Significant unconsolidated VIE assets	199,734	192,483
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	46,351	48,187
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	876	902
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	13,584	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	60,811	62,744
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	873	1,493
Consolidated VIE/SPE assets	873	883
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 investments
Variable Interest Entity
Total involvement with SPE assets	21,060	21,317
Consolidated VIE/SPE assets	3	3
Significant unconsolidated VIE assets	21,057	21,314
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,221	2,243
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	2,675	2,779
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,657	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	7,553	7,609
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	357	368
Consolidated VIE/SPE assets	79	86
Significant unconsolidated VIE assets	278	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	13	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	41	37
Investment funds
Variable Interest Entity
Total involvement with SPE assets	559	545
Consolidated VIE/SPE assets	56	70
Significant unconsolidated VIE assets	503	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	11	10
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	93	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	$ 108	$ 108