Investment Securities - Investment Securities in an Unrealized Loss Position (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
AFS securities
AFS: Fair Value, Less than 12 Months	$ 27,202	$ 8,824
AFS: Fair Value, 12 Months or Longer	1,938	2,358
AFS: Fair Value, Total	29,140	11,182
AFS: Gross Unrealized Losses, Less than 12 Months	341	31
AFS: Gross Unrealized Losses, 12 Months or Longer	13	30
AFS: Gross Unrealized Losses, Total	354	61
U.S. Treasury securities
AFS securities
AFS: Fair Value, Less than 12 Months	12,312	151
AFS: Fair Value, Total	12,312	151
AFS: Gross Unrealized Losses, Less than 12 Months	40	0
AFS: Gross Unrealized Losses, Total	40	0
U.S. agency securities
AFS securities
AFS: Fair Value, Less than 12 Months	10,657	5,808
AFS: Fair Value, 12 Months or Longer	994	1,168
AFS: Fair Value, Total	11,651	6,976
AFS: Gross Unrealized Losses, Less than 12 Months	208	22
AFS: Gross Unrealized Losses, 12 Months or Longer	2	3
AFS: Gross Unrealized Losses, Total	210	25
Agency CMBS
AFS securities
AFS: Fair Value, Less than 12 Months	3,924	2,779
AFS: Fair Value, 12 Months or Longer	45	46
AFS: Fair Value, Total	3,969	2,825
AFS: Gross Unrealized Losses, Less than 12 Months	92	9
AFS: Gross Unrealized Losses, 12 Months or Longer	0	0
AFS: Gross Unrealized Losses, Total	92	9
Corporate bonds
AFS securities
AFS: Fair Value, Less than 12 Months	52	0
AFS: Fair Value, 12 Months or Longer	10	31
AFS: Fair Value, Total	62	31
AFS: Gross Unrealized Losses, Less than 12 Months	1	0
AFS: Gross Unrealized Losses, 12 Months or Longer	0	0
AFS: Gross Unrealized Losses, Total	1	0
State and municipal securities
AFS securities
AFS: Fair Value, Less than 12 Months	14	86
AFS: Fair Value, 12 Months or Longer	0	36
AFS: Fair Value, Total	14	122
AFS: Gross Unrealized Losses, Less than 12 Months	0	0
AFS: Gross Unrealized Losses, 12 Months or Longer	0	1
AFS: Gross Unrealized Losses, Total	0	1
FFELP Student Loan ABS
AFS securities
AFS: Fair Value, Less than 12 Months	243	0
AFS: Fair Value, 12 Months or Longer	889	1,077
AFS: Fair Value, Total	1,132	1,077
AFS: Gross Unrealized Losses, Less than 12 Months	0	0
AFS: Gross Unrealized Losses, 12 Months or Longer	11	26
AFS: Gross Unrealized Losses, Total	$ 11	$ 26

No definition available.

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionQualitativeDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922890-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27337-111563

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922890-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922888-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=ms_AssetBackedSecuritiesFederalFamilyEducationLoanProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: