SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Funding Commitments (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 16,802	$ 16,781
Liquidity facilities
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	4,085	4,262
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	4,085	4,262
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	12,717	12,519
Loan / equity commitments | Asset-based financing
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	9,873	9,757
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	2,809	2,738
Loan / equity commitments | Investment funds
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	20	1
Loan / equity commitments | Other
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 15	$ 23