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Organization (Details)	12 Months Ended
	Dec. 31, 2014 item entity property	Dec. 31, 2013 entity	Dec. 31, 2012 entity
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	5fsp_MortgageLoanOnRealEstateNumberOfWhollyOwnedSubsidiariesOfSponsoredREITsToWhomLoanIsProvided
Number of REITs in which the entity holds non-controlling common stock interest	10fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingCommonStockInterest
Number of REITs in which the entity holds non-controlling preferred stock interest	2fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingPreferredStockInterest
Number of Sponsored REITs	10fsp_NumberOfSponsoredREITs	14fsp_NumberOfSponsoredREITs	15fsp_NumberOfSponsoredREITs
Number of properties	38fsp_NumberOfRealEstatePropertiesHeldForSale
Mortgage loan secured by property
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	1fsp_NumberOfPromissoryNotesSecuredByMortgagesOnRealEstateOwnedBySponsoredREITs / us-gaap_LongtermDebtTypeAxis = us-gaap_MortgagesMember
Secured construction loan
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	1fsp_MortgageLoanOnRealEstateNumberOfWhollyOwnedSubsidiariesOfSponsoredREITsToWhomLoanIsProvided / us-gaap_LongtermDebtTypeAxis = us-gaap_ConstructionLoansMember
Secured revolving lines of credit
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	3fsp_MortgageLoanOnRealEstateNumberOfSponsoredREITsToWhomLoanIsProvided / us-gaap_LongtermDebtTypeAxis = fsp_RevolvingLineOfCreditMember
FSP Investments LLC
Organization
Ownership interest (as a percent)	100.00%fsp_OwnershipInterestPercentageOfParentInConsolidatedEntities / dei_LegalEntityAxis = fsp_FSPInvestmentsLLCMember
FSP Property Management LLC
Organization
Ownership interest (as a percent)	100.00%fsp_OwnershipInterestPercentageOfParentInConsolidatedEntities / dei_LegalEntityAxis = fsp_FSPPropertyManagementLLCMember
FSP Holdings LLC
Organization
Ownership interest (as a percent)	100.00%fsp_OwnershipInterestPercentageOfParentInConsolidatedEntities / dei_LegalEntityAxis = fsp_FSPHoldingsLLCMember
FSP Protective TRS Corp.
Organization
Ownership interest (as a percent)	100.00%fsp_OwnershipInterestPercentageOfParentInConsolidatedEntities / dei_LegalEntityAxis = fsp_FSPProtectiveTRSCorpMember

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

Represents the number of real estate properties held for sale as of the balance sheet date.

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Definition

Number of sponsored real estate investment trusts in which the entity has an interest.

+ References

No definition available.

+ Details

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

X

- Details

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

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Details

Name:	dei_LegalEntityAxis=fsp_FSPHoldingsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=fsp_FSPInvestmentsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=fsp_FSPPropertyManagementLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=fsp_FSPProtectiveTRSCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_LongtermDebtTypeAxis=fsp_RevolvingLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: