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Organization (Details) | Dec. 31, 2016 ft² property entity item | Dec. 31, 2015 entity | Dec. 31, 2014 entity
--- | --- | --- | ---
Properties |  |  |
Rentable square feet | ft² | 1,099,000 |  |
Number of REITs in which the entity holds non-controlling common stock interest | entity | 7 | 9 | 10
Number of REITs in which the entity holds non-controlling preferred stock interest | entity | 2 |  |
Number of Sponsored REITs | entity | 7 | 9 | 10
Number of properties | property | 36 |  |
Number of properties in redevelopment | 1 |  |
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | 5 |  |
Mortgage loan secured by property |  |  |
Properties |  |  |
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | 2 |  |
Secured revolving lines of credit |  |  |
Properties |  |  |
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | 3 |  |
FSP Investments LLC |  |  |
Properties |  |  |
Ownership interest (as a percent) | 100.00% |  |
FSP Property Management LLC |  |  |
Properties |  |  |
Ownership interest (as a percent) | 100.00% |  |
FSP Holdings LLC |  |  |
Properties |  |  |
Ownership interest (as a percent) | 100.00% |  |
FSP Protective TRS Corp. |  |  |
Properties |  |  |
Ownership interest (as a percent) | 100.00% |  |

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 properties in redevelopment. + References No definition available. + Details
Name: | fsp_NumberOfPropertiesInRedevelopment
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
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Namespace Prefix: | fsp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the number of sponsored REITs the Company held an interest in at period end. + References No definition available. + Details
Name: | fsp_NumberOfSponsoredReitsCompanyHeldInterestIn
Namespace Prefix: | fsp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the ownership interest percentage of the parent in consolidated entities. + References No definition available. + Details
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Namespace Prefix: | fsp_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Area of a real estate property. + References No definition available. + Details
Name: | us-gaap_AreaOfRealEstateProperty
Namespace Prefix: | us-gaap_
Data Type: | num:areaItemType
Balance Type: | na
Period Type: | instant

X
- Definition The number of real estate properties owned as of the balance sheet date. + References No definition available. + Details
Name: | us-gaap_NumberOfRealEstateProperties
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=us-gaap_MortgagesMember
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: | 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_FSPHoldingsLLCMember
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: |