SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Funding Commitments (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 16,476	$ 16,883
Liquidity facilities
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	417
Liquidity facilities | Non-agency-sponsored mortgage securitizations
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Liquidity facilities | Citi-administered asset-backed commercial paper conduits
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Liquidity facilities | Asset-backed securities
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Liquidity facilities | Municipal securities tender option bond trusts (TOBs)
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	417
Liquidity facilities | Municipal investments
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Liquidity facilities | Investment funds
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Liquidity facilities | Other
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Loan / equity commitments
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	16,476	16,466
Loan / equity commitments | Non-agency-sponsored mortgage securitizations
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	229	129
Loan / equity commitments | Citi-administered asset-backed commercial paper conduits
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	38	0
Loan / equity commitments | Asset-backed securities
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	13,642	13,655
Loan / equity commitments | Municipal securities tender option bond trusts (TOBs)
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
Loan / equity commitments | Municipal investments
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	2,465	2,587
Loan / equity commitments | Investment funds
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	102	95
Loan / equity commitments | Other
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 0	$ 0