SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Variable Interest Entity
Total involvement with SPE assets	$ 512,917	$ 500,212
Consolidated VIE/SPE assets	57,258	58,637
Significant unconsolidated VIE assets	455,659	441,575
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	43,977	41,528
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,705	4,756
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	17,871	17,266
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	116	268
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	66,669	63,818
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	100,000	100,000
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	498	497
Venture capital investments
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	54,800	55,600
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	31,081	31,518
Consolidated VIE/SPE assets	31,081	31,518
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	116,330	113,641
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	116,330	113,641
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1,653	1,582
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	52	43
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	1,705	1,625
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	58,449	60,851
Consolidated VIE/SPE assets	605	632
Significant unconsolidated VIE assets	57,844	60,219
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,426	2,479
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	11	5
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	2,437	2,484
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	13,588	14,018
Consolidated VIE/SPE assets	13,588	14,018
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	7,315	8,302
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	7,315	8,302
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,608	2,636
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	2,608	2,636
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	260,573	246,632
Consolidated VIE/SPE assets	10,614	11,085
Significant unconsolidated VIE assets	249,959	235,547
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	34,590	32,242
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	1,112	1,139
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	12,288	12,189
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	47,990	45,570
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	3,410	3,251
Consolidated VIE/SPE assets	886	905
Significant unconsolidated VIE assets	2,524	2,346
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	21	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	1,736	1,498
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	1,757	1,500
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	20,961	20,597
Consolidated VIE/SPE assets	3	3
Significant unconsolidated VIE assets	20,958	20,594
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,611	2,512
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,593	3,617
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,821	3,562
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	10,025	9,691
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	832	904
Consolidated VIE/SPE assets	392	297
Significant unconsolidated VIE assets	440	607
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	66	75
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	63	224
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	129	299
Investment funds
Variable Interest Entity
Total involvement with SPE assets	378	498
Consolidated VIE/SPE assets	89	179
Significant unconsolidated VIE assets	289	319
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	15	12
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	1	1
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	18	13
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