SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Variable Interest Entity
Total involvement with SPE assets	$ 302,510	$ 401,822
Consolidated VIE / SPE assets	72,169	76,291
Significant unconsolidated VIE assets	230,341	325,531
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	26,952	28,523
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	4,302	4,219
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	10,687	10,422
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	184	190
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	42,125	43,354
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	46,993	50,171
Consolidated VIE / SPE assets	46,993	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-backed securities - U.S. agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	117,210	214,458
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	117,210	214,458
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,766	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	2,840	3,930
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	20,164	15,965
Consolidated VIE / SPE assets	1,037	1,092
Significant unconsolidated VIE assets	19,127	14,873
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	284	312
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	35	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	320	348
Citi-administered asset-backed commercial paper conduits (ABCP)
Variable Interest Entity
Total involvement with SPE assets	19,120	19,693
Consolidated VIE / SPE assets	19,120	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	18,246	18,886
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	18,246	18,886
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	5,071	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	64	62
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	5,135	5,190
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	50,939	53,168
Consolidated VIE / SPE assets	742	733
Significant unconsolidated VIE assets	50,197	52,435
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	15,707	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	4,923	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	21,213	21,943
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	6,927	7,070
Consolidated VIE / SPE assets	2,659	2,843
Significant unconsolidated VIE assets	4,268	4,227
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	8	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	2,914	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	2,922	2,882
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	18,463	17,679
Consolidated VIE / SPE assets	15	14
Significant unconsolidated VIE assets	18,448	17,665
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,507	2,441
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,639	3,578
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,561	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	8,707	8,599
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	1,504	515
Consolidated VIE / SPE assets	800	371
Significant unconsolidated VIE assets	704	144
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	462	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	208	0
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	670	52
Investment funds
Variable Interest Entity
Total involvement with SPE assets	2,177	2,788
Consolidated VIE / SPE assets	767	767
Significant unconsolidated VIE assets	1,410	2,021
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	32	32
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	34	120
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	15	27
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	1	3
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	82	182
Other
Variable Interest Entity
Total involvement with SPE assets	767	1,429
Consolidated VIE / SPE assets	36	607
Significant unconsolidated VIE assets	731	822
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	115	116
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	11	11
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	66	58
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	44	43
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	236	228
Mortgage-backed securities
Funded and Unfunded Exposure
Outstanding balance of mortgage loans securitized	$ 10,000	$ 10,000