Organization (Details)	12 Months Ended
Dec. 31, 2011 note
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	7
Number of corporations organized to operate as real estate investment trusts (REITs)	16
Number of REITs in which the entity holds non-controlling preferred stock interest	3
Commercial real estate:
Number of properties	36
Mortgage loan with a revolving line of credit component
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	1
Revolving lines of credit
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	5
Construction loan
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	1
FSP Investments LLC
Organization
Ownership interest (as a percent)	100.00%
FSP Property Management LLC
Organization
Ownership interest (as a percent)	100.00%
FSP Holdings LLC
Organization
Ownership interest (as a percent)	100.00%
FSP Protective TRS Corp.
Organization
Ownership interest (as a percent)	100.00%

Represents the number of sponsored REITs of the entity to which loans have been provided.

No definition available.

Name:	fsp_MortgageLoanOnRealEstateNumberOfSponsoredREITsToWhomLoanIsProvided
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Represents the number of wholly-owned subsidiaries of the sponsored REITs of the entity to whom loan has been provided.

No definition available.

Name:	fsp_MortgageLoanOnRealEstateNumberOfWhollyOwnedSubsidiariesOfSponsoredREITsToWhomLoanIsProvided
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Represents the number of promissory notes secured by mortgages on real estate owned by Sponsored REITs.

No definition available.

Name:	fsp_NumberOfPromissoryNotesSecuredByMortgagesOnRealEstateOwnedBySponsoredREITs
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of entities in which the reporting entity has a non-controlling common stock interest.

No definition available.

Name:	fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingCommonStockInterest
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of entities in which the reporting entity has a non-controlling preferred stock interest.

No definition available.

Name:	fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingPreferredStockInterest
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Name:	fsp_OrganizationAndConsolidationLineItems
Namespace Prefix:	fsp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Represents the ownership interest percentage of the parent in consolidated entities.

No definition available.

Name:	fsp_OwnershipInterestPercentageOfParentInConsolidatedEntities
Namespace Prefix:	fsp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The number of real estate properties owned as of the balance sheet date.

No definition available.

Name:	us-gaap_NumberOfRealEstateProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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