SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Funding Commitments (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 16,094	$ 16,157
Liquidity facilities
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	3,125	3,544
Liquidity facilities | Asset-based financing
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	3,125	3,544
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	12,969	12,613
Loan / equity commitments | Asset-based financing
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	9,863	9,524
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	3,030	3,034
Loan / equity commitments | Investment funds
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	16	16
Loan / equity commitments | Other
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 60	$ 39