SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Variable Interest Entity
Total involvement with SPE assets	$ 401,689	$ 361,936
Consolidated VIE/SPE assets	64,350	69,944
Significant unconsolidated VIE assets	337,339	291,992
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	36,184	34,043
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,923	4,637
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	16,802	16,781
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	80	73
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	57,989	55,534
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	43,090	46,232
Consolidated VIE/SPE assets	43,090	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	114,735	116,563
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	114,735	116,563
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,920	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	71	60
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,991	3,098
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	35,021	30,886
Consolidated VIE/SPE assets	1,337	1,498
Significant unconsolidated VIE assets	33,684	29,388
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	757	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	758	432
Citi-administered asset-backed commercial paper conduits
Variable Interest Entity
Total involvement with SPE assets	16,419	18,750
Consolidated VIE/SPE assets	16,419	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	19,062	21,837
Consolidated VIE/SPE assets	0	0
Significant unconsolidated VIE assets	19,062	21,837
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4,945	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	8	9
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	4,953	5,900
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	144,436	99,433
Consolidated VIE/SPE assets	660	628
Significant unconsolidated VIE assets	143,776	98,805
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	24,532	21,640
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	842	715
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	9,873	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	35,247	32,112
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	7,841	7,998
Consolidated VIE/SPE assets	1,623	1,776
Significant unconsolidated VIE assets	6,218	6,222
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	13	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	4,085	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	4,098	4,271
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	18,479	18,044
Consolidated VIE/SPE assets	0	3
Significant unconsolidated VIE assets	18,479	18,041
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,620	2,813
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,081	3,922
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,809	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	9,510	9,473
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	1,183	858
Consolidated VIE/SPE assets	955	614
Significant unconsolidated VIE assets	228	244
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	169	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	169	174
Investment funds
Variable Interest Entity
Total involvement with SPE assets	1,054	1,272
Consolidated VIE/SPE assets	264	440
Significant unconsolidated VIE assets	790	832
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	15	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	20	1
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	35	14
Other
Variable Interest Entity
Total involvement with SPE assets	369	63
Consolidated VIE/SPE assets	2	3
Significant unconsolidated VIE assets	367	60
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	213	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	15	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	228	60
Mortgage-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 7,000	$ 7,000