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Related Party Transactions and Investments in Non-Consolidated Entities - Investment in Sponsored REITs (Details) - entity | Sep. 30, 2021 | Dec. 31, 2020
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Related Party Transactions and Investments in Non-Consolidated Entities |  |
Number of REITs in which the entity holds non-controlling common stock interest | 2 | 2

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

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