Represents the number of properties sold which are primarily related to disposition and classified as held-for-sale.

Name:	fsp_NumberOfPropertiesPrimarilyRelatedToDispositionClassifiedAsHeldForSaleSold
Namespace Prefix:	fsp
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of properties sold primarily related to disposition.

Name:	fsp_NumberOfPropertiesPrimarilyRelatedToDispositionSold
Namespace Prefix:	fsp
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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

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.

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.

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

The number of reportable segments of the entity.

Name:	fsp_ReportableSegmentsNumber
Namespace Prefix:	fsp
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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

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

Name:	us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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

The amount of square footage related to a real estate property.

Name:	us-gaap_SquareFootageOfRealEstateProperty
Namespace Prefix:	us-gaap
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details)	9 Months Ended
	Sep. 30, 2011 sqft Segment Y Property D Entity	Sep. 30, 2010 sqft Property Entity	Dec. 31, 2010 Entity Property
Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards
Number of corporations organized to operate as real estate investment trusts ("REITs") in which the entity holds non-controlling common stock interest	17
Number of REITs in which the entity holds non-controlling preferred stock interest	3
Number of business segments	2
Number of properties sold primarily related to disposition	2
Number of properties classified as held-for-sale at December 31, 2010 sold			1
Commercial real estate:
Number of properties	35	33
Rentable square feet (in square feet)	6,929,891	6,417,299
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%