SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Variable Interest Entity
Total involvement with SPE assets	$ 512,202	$ 485,411
Consolidated VIE/SPE assets	56,487	60,056
Significant unconsolidated VIE assets	455,715	425,355
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	39,890	36,581
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,992	5,643
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	15,335	13,783
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	107	118
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	60,324	56,125
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	109,000	78,000
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	507	425
Venture capital investments
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	59,600	57,000
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	30,948	32,420
Consolidated VIE/SPE assets	30,948	32,420
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	114,664	123,999
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	114,664	123,999
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1,533	1,948
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	50	61
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	1,583	2,009
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	57,157	46,132
Consolidated VIE/SPE assets	773	939
Significant unconsolidated VIE assets	56,384	45,193
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,893	2,550
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	5	2
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	0	1
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,898	2,553
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	14,566	16,730
Consolidated VIE/SPE assets	14,566	16,730
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	11,430	18,332
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	11,430	18,332
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3,529	4,273
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	3,529	4,273
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	256,827	222,274
Consolidated VIE/SPE assets	8,657	8,069
Significant unconsolidated VIE assets	248,170	214,205
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	29,196	25,153
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	1,195	1,587
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	10,255	9,114
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	40,646	35,854
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	3,302	3,349
Consolidated VIE/SPE assets	915	835
Significant unconsolidated VIE assets	2,387	2,514
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	1,523	1,611
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	1,523	1,611
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	21,914	20,335
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	21,914	20,335
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,662	2,569
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,797	4,056
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,539	3,041
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	9,998	9,666
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	911	1,352
Consolidated VIE/SPE assets	470	910
Significant unconsolidated VIE assets	441	442
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	75	88
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	56	56
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	131	144
Investment funds
Variable Interest Entity
Total involvement with SPE assets	483	488
Consolidated VIE/SPE assets	158	153
Significant unconsolidated VIE assets	325	335
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2	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	13	15
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	1	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	16	15
Other
Variable Interest Entity
Total involvement with SPE assets	0	0
Consolidated VIE/SPE assets	0	0
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-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 5,000	$ 5,200