SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Variable Interest Entity
Total involvement with SPE assets	$ 531,837	$ 485,411
Consolidated VIE/SPE assets	55,970	60,056
Significant unconsolidated VIE assets	475,867	425,355
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	38,920	36,581
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,989	5,643
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	16,109	13,783
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	258	118
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	60,276	56,125
Private equity
Variable Interest Entity
Significant unconsolidated VIE assets	115,000	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	56,600	57,000
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	30,343	32,420
Consolidated VIE/SPE assets	30,343	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	114,519	123,999
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	114,519	123,999
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1,537	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	47	61
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	1,584	2,009
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	59,584	46,132
Consolidated VIE/SPE assets	677	939
Significant unconsolidated VIE assets	58,907	45,193
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,327	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	1	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	2,328	2,553
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	12,460	16,730
Consolidated VIE/SPE assets	12,460	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	10,094	18,332
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	10,094	18,332
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3,254	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	3,254	4,273
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	279,044	222,274
Consolidated VIE/SPE assets	11,106	8,069
Significant unconsolidated VIE assets	267,938	214,205
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	29,033	25,153
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	1,283	1,587
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	11,008	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	41,324	35,854
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	3,320	3,349
Consolidated VIE/SPE assets	909	835
Significant unconsolidated VIE assets	2,411	2,514
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	16	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	1,529	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	1,545	1,611
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	21,078	20,335
Consolidated VIE/SPE assets	3	0
Significant unconsolidated VIE assets	21,075	20,335
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,677	2,569
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,706	4,056
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,558	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	9,941	9,666
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	872	1,352
Consolidated VIE/SPE assets	280	910
Significant unconsolidated VIE assets	592	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	206	56
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	281	144
Investment funds
Variable Interest Entity
Total involvement with SPE assets	523	488
Consolidated VIE/SPE assets	192	153
Significant unconsolidated VIE assets	331	335
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	1	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	5	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	19	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	$ 4,700	$ 5,200