SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Variable Interest Entity
Total involvement with SPE assets	$ 460,157	$ 361,936
Consolidated VIE / SPE assets	69,506	69,944
Significant unconsolidated VIE assets	390,651	291,992
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	34,320	34,043
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	5,425	4,637
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	16,157	16,781
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	74	73
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	55,976	55,534
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	43,534	46,232
Consolidated VIE / SPE assets	43,534	46,232
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 - U.S. agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	117,374	116,563
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	117,374	116,563
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,671	3,038
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	72	60
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,743	3,098
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	39,608	30,886
Consolidated VIE / SPE assets	1,187	1,498
Significant unconsolidated VIE assets	38,421	29,388
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	876	431
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	1	1
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	877	432
Citi-administered asset-backed commercial paper conduits (ABCP)
Variable Interest Entity
Total involvement with SPE assets	15,622	18,750
Consolidated VIE / SPE assets	15,622	18,750
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	17,395	21,837
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	17,395	21,837
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4,199	5,891
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	9
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	4,199	5,900
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	196,728	99,433
Consolidated VIE / SPE assets	6,139	628
Significant unconsolidated VIE assets	190,589	98,805
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	23,756	21,640
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	1,151	715
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	9,524	9,757
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	34,431	32,112
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	6,950	7,998
Consolidated VIE / SPE assets	1,458	1,776
Significant unconsolidated VIE assets	5,492	6,222
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4	9
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,544	4,262
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	4,271
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	20,312	18,044
Consolidated VIE / SPE assets	0	3
Significant unconsolidated VIE assets	20,312	18,041
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,636	2,813
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,274	3,922
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,034	2,738
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	9,944	9,473
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	1,455	858
Consolidated VIE / SPE assets	1,391	614
Significant unconsolidated VIE assets	64	244
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4	172
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	2
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	4	174
Investment funds
Variable Interest Entity
Total involvement with SPE assets	827	1,272
Consolidated VIE / SPE assets	174	440
Significant unconsolidated VIE assets	653	832
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	5	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	16	1
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	22	14
Other
Variable Interest Entity
Total involvement with SPE assets	352	63
Consolidated VIE / SPE assets	1	3
Significant unconsolidated VIE assets	351	60
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	169	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	39	23
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	208	60
Mortgage-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 6,000	$ 7,000