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Significant Accounting Policies - Basis, Estimates and Inv in non-consolidated REITs (Detail) - entity | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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Significant Accounting Policies |  |  |
Number of corporations organized to operate as real estate investment trusts (REITs) | 2 | 2 | 3
Number of REITs in which the entity holds non-controlling preferred 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

X
- Definition Represents the number of entities in which the reporting entity has a non-controlling preferred stock interest. + References No definition available. + Details
Name: | fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingPreferredStockInterest
Namespace Prefix: | fsp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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