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Organization (Details) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 property entity item | Dec. 31, 2016 entity | Dec. 31, 2015 entity
Organization |  |  |
Number of REITs in which the entity holds non-controlling common stock interest | entity | 6 | 7 | 9
Number of REITs in which the entity holds non-controlling preferred stock interest | entity | 2 |  |
Number of Sponsored REITs | entity | 6 | 7 | 9
Number of properties in redevelopment | 1 |  |
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | 4 |  |
Properties |  |  |
Number of properties | property | 34 |  |
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% |  |
Mortgage loan secured by property |  |  |
Organization |  |  |
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | 2 |  |
Secured revolving lines of credit |  |  |
Organization |  |  |
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | 2 |  |

X
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- Definition Represents the number of properties in redevelopment. + References No definition available. + Details
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- Definition Represents the number of entities in which the reporting entity has a non-controlling common stock interest. + References No definition available. + Details
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- Definition Represents the number of sponsored REITs the Company held an interest in at period end. + References No definition available. + Details
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- Definition Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 272 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055 + Details
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- Definition The parent entity's interest in net assets of the subsidiary, expressed as a percentage. + References No definition available. + Details
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- Details
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