Label	Element	Value
Risk Return Abstract	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Period End Date	dei_DocumentPeriodEndDate	Aug. 31, 2021
Registrant Name	dei_EntityRegistrantName	FEDERATED HERMES MUNICIPAL SECURITIES INCOME TRUST
CIK	dei_EntityCentralIndexKey	0000866700
Amendment Flag	dei_AmendmentFlag	false
Document Created Date	dei_DocumentCreationDate	Oct. 26, 2021
Effective Date	dei_DocumentEffectiveDate	Oct. 28, 2021
Prospectus Date	rr_ProspectusDate	Oct. 31, 2021
Entity Inv Company Type	dei_EntityInvCompanyType	N-1A
Federated Hermes Michigan Intermediate Municipal Fund \| A
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MMIFX
Federated Hermes Michigan Intermediate Municipal Fund \| IS
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MMFIX
Federated Hermes Municipal High Yield Advantage Fund \| A
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	FMOAX
Federated Hermes Municipal High Yield Advantage Fund \| B
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	FMOBX
Federated Hermes Municipal High Yield Advantage Fund \| C
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	FMNCX
Federated Hermes Municipal High Yield Advantage Fund \| F
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	FHTFX
Federated Hermes Municipal High Yield Advantage Fund \| IS
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	FMYIX
Federated Hermes Ohio Municipal Income Fund \| A
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	OMIAX
Federated Hermes Ohio Municipal Income Fund \| F
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	OMIFX
Federated Hermes Ohio Municipal Income Fund \| IS
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	OMIIX
Federated Hermes Pennsylvania Municipal Income Fund \| A
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	PAMFX
Federated Hermes Pennsylvania Municipal Income Fund \| IS
Risk Return Abstract	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	PAMIX

Boolean flag that is true when the XBRL content amends previously-filed or accepted submission.

No definition available.

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- Definition

The date the document was made available and submitted, in YYYY-MM-DD format. The date of submission, date of acceptance by the recipient, and the document effective date are all potentially different.

+ References

No definition available.

+ Details

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- Definition

The date when a document, upon receipt and acceptance, becomes officially effective, in YYYY-MM-DD format. Usually it is a system-assigned date time value, but it may be declared by the submitter in some cases.

+ References

No definition available.

+ Details

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- Definition

The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented. If there is no historical data in the report, use the filing date. The format of the date is YYYY-MM-DD.

+ References

No definition available.

+ Details

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- Definition

The type of document being provided (such as 10-K, 10-Q, 485BPOS, etc). The document type is limited to the same value as the supporting SEC submission type, or the word 'Other'.

+ References

No definition available.

+ Details

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- Definition

A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation 12B
-Number 240
-Section 12
-Subsection b-2

+ Details

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- Definition

One of: N-1A (Mutual Fund), N-1 (Open-End Separate Account with No Variable Annuities), N-2 (Closed-End Investment Company), N-3 (Separate Account Registered as Open-End Management Investment Company), N-4 (Variable Annuity UIT Separate Account), N-5 (Small Business Investment Company), N-6 (Variable Life UIT Separate Account), S-1 or S-3 (Face Amount Certificate Company), S-6 (UIT, Non-Insurance Product).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-T
-Number 232
-Section 313

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- Definition

The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation 12B
-Number 240
-Section 12
-Subsection b-2

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- Definition

Trading symbol of an instrument as listed on an exchange.

+ References

No definition available.

+ Details

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- Definition

The date of the prospectus.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form
-Number N-1A
-Chapter A
-Section 1
-Subsection a
-Paragraph 3

+ Details

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- References

No definition available.

+ Details

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