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Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details) | 9 Months Ended |
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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% |  |

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- Definition Represents the number of entities in which the reporting entity has a non-controlling common stock interest. + Details
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- Definition The number of reportable segments of the entity. + Details
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- Definition The number of real estate properties owned as of the balance sheet date. + Details
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- Details
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- Definition The amount of square footage related to a real estate property. + Details
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