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Related Party Transactions and Investments in Non-Consolidated Entities (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015 entity	Dec. 31, 2014 entity	Mar. 31, 2014 entity
Related Party Transactions and Investments in Non-Consolidated Entities
Sponsored REITs (in entities)	9fsp_NumberOfSponsoredREITs	10fsp_NumberOfSponsoredREITs	14fsp_NumberOfSponsoredREITs
Number of REITs in which the entity holds non-controlling preferred stock interest	2fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingPreferredStockInterest
Number of sponsored REITs having properties which were sold	1fsp_NumberOfSponsoredREITsHavingPropertiesWhichWereSold	4fsp_NumberOfSponsoredREITsHavingPropertiesWhichWereSold
Number of entities holding mortgage loans		2fsp_NumberOfEntitiesHoldingMortgageLoans
FSP Galleria North Corp.
Investment in Sponsored REITs
Repayment of principal		13,880,000us-gaap_RepaymentsOfDebt / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = fsp_FSPGalleriaNorthCorpMember
FSP Highland Place I Corp.
Investment in Sponsored REITs
Repayment of principal		3,395,000us-gaap_RepaymentsOfDebt / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = fsp_FSPHighLandPlaceICorpMember

X

- Definition

Represents the number of entities holding mortgage loans.

+ References

No definition available.

+ Details

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- 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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X

- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

Represents the number of sponsored REITs having properties which were sold.

+ References

No definition available.

+ Details

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- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

No definition available.

+ Details

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- Details

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- Details

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