Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Mar. 31, 2011
Registrant Name	dei_EntityRegistrantName	FEDERATED MUNICIPAL SECURITIES FUND INC
Central Index Key	dei_EntityCentralIndexKey	0000201801
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	May 27, 2011
Document Effective Date	dei_DocumentEffectiveDate	May 27, 2011
Prospectus Date	rr_ProspectusDate	May 31, 2011

If the value is true, then the document as an amendment to previously-filed/accepted document.

Name:	dei_AmendmentFlag
Namespace Prefix:	dei
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ Details

Name:	dei_DocumentCreationDate
Namespace Prefix:	dei
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ Details

Name:	dei_DocumentEffectiveDate
Namespace Prefix:	dei
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements this will be the filing date. The format of the date is CCYY-MM-DD.

+ Details

Name:	dei_DocumentPeriodEndDate
Namespace Prefix:	dei
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The type of document being provided (such as 10-K, 10-Q, N-1A, etc). The document type should be limited to the same value as the supporting SEC submission type. The acceptable values are as follows: S-1, S-3, S-4, S-11, F-1, F-3, F-4, F-9, F-10, 6-K, 8-K, 10, 10-K, 10-Q, 20-F, 40-F, N-1A, 485BPOS, NCSR, N-Q, and Other.

+ Details

Name:	dei_DocumentType
Namespace Prefix:	dei
Data Type:	us-types:SECReportItemType
Balance Type:	na
Period Type:	duration

- 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 12b
-Subsection 1

+ Details

Name:	dei_EntityCentralIndexKey
Namespace Prefix:	dei
Data Type:	us-types:centralIndexKeyItemType
Balance Type:	na
Period Type:	duration

- 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 12b
-Subsection 1

+ Details

Name:	dei_EntityRegistrantName
Namespace Prefix:	dei
Data Type:	xbrli:normalizedStringItemType
Balance Type:	na
Period Type:	duration

- Definition

The date of the prospectus.

+ References

Reference 1: 1
-Publisher SEC
-Name Form
-Number N-1A
-Chapter A
-Section 1
-Paragraph 3
-Subsection a

+ Details

Name:	rr_ProspectusDate
Namespace Prefix:	rr
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	rr_RiskReturnAbstract
Namespace Prefix:	rr
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration