SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Variable Interest Entity
Total involvement with SPE assets	$ 462,267	$ 511,990
Consolidated VIE/SPE assets	63,609	62,201
Significant unconsolidated VIE assets	398,658	449,789
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	51,791	50,861
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,819	4,170
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	16,448	15,322
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	139	61
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	72,197	70,414
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	6,000	69,000
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	283	498
Venture capital investments
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	13,800	33,600
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	31,203	32,021
Consolidated VIE/SPE assets	31,203	32,021
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	125,485	117,358
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	125,485	117,358
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,149	2,052
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	139	48
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,288	2,100
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	64,111	67,704
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	64,111	67,704
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3,133	3,294
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	130	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	3,263	3,294
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	20,852	19,621
Consolidated VIE/SPE assets	20,852	19,621
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	5,767	7,600
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	5,767	7,600
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,455	2,601
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	2,455	2,601
Asset-backed securities
Variable Interest Entity
Total involvement with SPE assets	190,782	242,348
Consolidated VIE/SPE assets	10,652	9,672
Significant unconsolidated VIE assets	180,130	232,676
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	41,613	40,121
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	927	1,022
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	12,775	10,726
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	55,315	51,869
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	1,410	2,155
Consolidated VIE/SPE assets	723	672
Significant unconsolidated VIE assets	687	1,483
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	5	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	519	1,108
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	524	1,110
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	21,657	22,167
Consolidated VIE/SPE assets	3	3
Significant unconsolidated VIE assets	21,654	22,164
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,356	2,731
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	2,884	3,143
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,934	3,420
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	8,174	9,294
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	496	482
Consolidated VIE/SPE assets	106	121
Significant unconsolidated VIE assets	390	361
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	75	58
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	13
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	75	71
Investment funds
Variable Interest Entity
Total involvement with SPE assets	504	534
Consolidated VIE/SPE assets	70	91
Significant unconsolidated VIE assets	434	443
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	5	2
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	8	5
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	90	68
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	$ 103	$ 75