Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details)	Sep. 30, 2012 entity note item sqft property	Sep. 30, 2011 property sqft
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	8
Number of corporations organized to operate as real estate investment trusts (REITs)	16
Number of properties held for sale	1
Number of REITs in which the entity holds non-controlling preferred stock interest	3
Commercial real estate:
Number of properties	37	35
Rentable square feet	7,439,195	6,929,891
Mortgage 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 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 real estate properties held for sale as of the balance sheet date.

No definition available.

Name:	fsp_NumberOfRealEstatePropertiesHeldForSale
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

Area of a real estate property.

No definition available.

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
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