SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity
Total involvement with SPE assets	$ 319,569	$ 401,822
Consolidated VIE / SPE assets	76,394	76,291
Significant unconsolidated VIE assets	243,175	325,531
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	31,331	28,523
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,239	4,219
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	11,304	10,422
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	140	190
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	47,014	43,354
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	50,795	50,171
Consolidated VIE / SPE assets	50,795	50,171
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	116,610	214,458
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	116,610	214,458
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,647	3,852
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	74	78
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	2,721	3,930
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	22,251	15,965
Consolidated VIE / SPE assets	2,035	1,092
Significant unconsolidated VIE assets	20,216	14,873
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	330	312
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	35
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	331	348
Citi-administered asset-backed commercial paper conduits (ABCP)
Variable Interest Entity
Total involvement with SPE assets	19,282	19,693
Consolidated VIE / SPE assets	19,282	19,693
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	20,588	18,886
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	20,588	18,886
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	5,956	5,128
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	9	62
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	5,965	5,190
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	60,472	53,168
Consolidated VIE / SPE assets	633	733
Significant unconsolidated VIE assets	59,839	52,435
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	19,478	16,553
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	583	475
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	5,878	4,915
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	25,939	21,943
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	6,925	7,070
Consolidated VIE / SPE assets	2,166	2,843
Significant unconsolidated VIE assets	4,759	4,227
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	138	40
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	3,035	2,842
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	3,173	2,882
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	19,119	17,679
Consolidated VIE / SPE assets	7	14
Significant unconsolidated VIE assets	19,112	17,665
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,709	2,441
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,640	3,578
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,344	2,580
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	8,693	8,599
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	958	515
Consolidated VIE / SPE assets	824	371
Significant unconsolidated VIE assets	134	144
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	32	49
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	9	3
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	41	52
Investment funds
Variable Interest Entity
Total involvement with SPE assets	1,892	2,788
Consolidated VIE / SPE assets	616	767
Significant unconsolidated VIE assets	1,276	2,021
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	14	32
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	7	120
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	13	27
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	0	3
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	34	182
Other
Variable Interest Entity
Total involvement with SPE assets	677	1,429
Consolidated VIE / SPE assets	36	607
Significant unconsolidated VIE assets	641	822
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	27	116
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	9	11
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	34	58
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	47	43
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	117	228
Mortgage-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 9,000	$ 10,000