SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Schedule of Variable Interest Entities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Variable Interest Entity
Total involvement with SPE assets	$ 409,422	$ 418,045
Consolidated VIE / SPE assets	77,956	84,794
Significant unconsolidated VIE assets	331,466	333,251
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	30,874	31,070
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,922	3,448
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	10,364	9,153
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	651	670
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	45,811	44,341
Credit card securitizations
Variable Interest Entity
Total involvement with SPE assets	50,416	54,916
Consolidated VIE / SPE assets	50,416	54,916
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	217,482	217,291
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	217,482	217,291
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	3,678	3,571
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	82	95
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	3,760	3,666
Mortgage securitizations - Non-agency-sponsored
Variable Interest Entity
Total involvement with SPE assets	15,257	13,036
Consolidated VIE / SPE assets	1,244	1,586
Significant unconsolidated VIE assets	14,013	11,450
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	232	527
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	38	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	271	528
Citi-administered asset-backed commercial paper conduits (ABCP)
Variable Interest Entity
Total involvement with SPE assets	20,324	21,280
Consolidated VIE / SPE assets	20,324	21,280
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	18,592	16,719
Consolidated VIE / SPE assets	0	0
Significant unconsolidated VIE assets	18,592	16,719
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	4,752	3,150
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	83	86
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	4,835	3,236
Asset-based financing
Variable Interest Entity
Total involvement with SPE assets	58,084	58,862
Consolidated VIE / SPE assets	1,231	1,364
Significant unconsolidated VIE assets	56,853	57,498
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	19,508	21,270
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	456	269
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	5,193	3,616
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	437	436
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	25,594	25,591
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Total involvement with SPE assets	7,289	8,572
Consolidated VIE / SPE assets	2,980	3,830
Significant unconsolidated VIE assets	4,309	4,742
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	161	2
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	0
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,672	3,100
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	2,833	3,102
Municipal investments
Variable Interest Entity
Total involvement with SPE assets	17,371	20,290
Consolidated VIE / SPE assets	17	44
Significant unconsolidated VIE assets	17,354	20,246
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	2,306	2,196
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	3,272	2,487
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	2,321	2,335
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	7,899	7,018
Client intermediation
Variable Interest Entity
Total involvement with SPE assets	517	434
Consolidated VIE / SPE assets	337	335
Significant unconsolidated VIE assets	180	99
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	53	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	0	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	53	49
Investment funds
Variable Interest Entity
Total involvement with SPE assets	2,744	1,730
Consolidated VIE / SPE assets	788	842
Significant unconsolidated VIE assets	1,956	888
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	35	13
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	156	138
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	59	102
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	3	0
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	253	253
Other
Variable Interest Entity
Total involvement with SPE assets	1,346	4,915
Consolidated VIE / SPE assets	619	597
Significant unconsolidated VIE assets	727	4,318
Funded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, debt investments	149	292
Maximum exposure to loss in significant unconsolidated VIEs, equity investments	0	554
Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs, funding commitments	119	0
Maximum exposure to loss in significant unconsolidated VIEs, guarantees and derivatives	45	52
Funded and Unfunded Exposure
Maximum exposure to loss in significant unconsolidated VIEs	313	898
Mortgage-backed securities
Funded and Unfunded Exposure
Private label mortgage-backed securities, outstanding	10,000	12,000
Citicorp
Variable Interest Entity
Total involvement with SPE assets	$ 390,900	$ 383,200