Commitments, Guarantees and Contingencies - Narrative (Details) € in Millions, $ in Millions	Mar. 31, 2021 USD ($)	Mar. 31, 2021 EUR (€)	Dec. 31, 2020 USD ($)
Loss Contingencies
Derivative asset	$ 17,561,000		$ 15,706,000
Financial Guaranty Insurance Company v. Morgan Stanley ABS Capital I Inc. et al. (Trust 2007-NC4) | Pending Litigation
Loss Contingencies
Original principal amount of debt instrument	876
Deutsche Bank National Trust Company solely in its capacity as Trustee of the Morgan Stanley ABS Capital I Inc. Trust 2007-NC4 v. Morgan Stanley Mortgage Capital Holdings LLC as Successor-by-Merger to Morgan Stanley Mortgage Capital Inc., and Morgan Stanley ABS Capital I Inc. | Pending Litigation | Maximum
Loss Contingencies
Estimate of possible loss	277
Case Number 15/3637 and Case Number 15/4353 | Pending Litigation
Loss Contingencies
Withholding tax credit set-off	145	€ 124
US Bank National Association 2007-2AX v. Morgan Stanley Mortgage Capital Holdings LLC | Pending Litigation
Loss Contingencies
Original principal amount of debt instrument	$ 1,050