SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Funding Commitments (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 15,315	$ 13,783
Liquidity facilities
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	1,557	1,611
Liquidity facilities | Non-agency-sponsored mortgage securitizations
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	0	0
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	1,557	1,611
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	13,758	12,172
Loan / equity commitments | Non-agency-sponsored mortgage securitizations
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	5	2
Loan / equity commitments | Asset-based financing
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	10,626	9,114
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	3,063	3,041
Loan / equity commitments | Investment funds
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	14	15
Loan / equity commitments | Other
Funding Commitments for Significant Unconsolidated VIEs
Notional amount	$ 50	$ 0