SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Funding Commitments (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 9,179	$ 8,313
Liquidity facilities Citigroup
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	3,141	3,675
Liquidity facilities Citigroup | Asset-based financing
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	5	5
Liquidity facilities Citigroup | Municipal securities tender option bond trusts (TOBs)
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	3,136	3,670
Liquidity facilities Citigroup | Municipal investments
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Liquidity facilities Citigroup | Investment funds
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Liquidity facilities Citigroup | Other
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Loan / equity commitments Citigroup
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	6,038	4,638
Loan / equity commitments Citigroup | Asset-based financing
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	3,261	2,266
Loan / equity commitments Citigroup | Municipal securities tender option bond trusts (TOBs)
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Loan / equity commitments Citigroup | Municipal investments
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	2,651	2,225
Loan / equity commitments Citigroup | Investment funds
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	104	124
Loan / equity commitments Citigroup | Other
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 22	$ 23