SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Funding Commitments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 16,781	$ 11,304
Liquidity facilities Citigroup
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	4,262	3,035
Liquidity facilities Citigroup | Asset-based financing
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Liquidity facilities Citigroup | Municipal securities tender option bond trusts (TOBs)
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	4,262	3,035
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	12,519	8,269
Loan / equity commitments Citigroup | Asset-based financing
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	9,757	5,878
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	2,738	2,344
Loan / equity commitments Citigroup | Investment funds
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	1	13
Loan / equity commitments Citigroup | Other
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 23	$ 34