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Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details 2) (USD $) In Millions, unless otherwise specified |  |  | 0 Months Ended
--- | --- | --- | ---
Mar. 31, 2013 property sqft | Mar. 31, 2012 sqft property | Mar. 08, 2013 Office property in Atlanta, Georgia sqft
Commercial real estate: |  |  |
Number of properties | 37 | 36 |
Rentable square feet | 7,856,859 | 7,052,068 | 621,007
Purchase price of property acquired in Atlanta, Georgia |  |  | $ 157.9

X
- Definition Represents the aggregate purchase price excluding closing costs and adjustments of real estate properties acquired by the entity during the period. + References No definition available. + Details
Name: | fsp_RealEstateAggregatePurchasePrice
Namespace Prefix: | fsp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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
- Details
Name: | us-gaap_RealEstateInvestmentsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration