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Significant Accounting Policies - Rent Receivables (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019
Tenant Rent Receivables and Straight-line Rent Receivable |  |
Period within which tenant rent receivables are expected to be collected | 1 year |
Straight-line rent receivable | $ 67.8 | $ 66.9

X
- Definition Represents deferred rent receivables net including amounts for assets held for sale. + References No definition available. + Details
Name: | fsp_DeferredRentReceivableNetIncludingAmountsForAssetsHeldForSale
Namespace Prefix: | fsp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Represents the maximum period within which receivables are expected to be collected. + References No definition available. + Details
Name: | fsp_TenantRentReceivablesCollectionPeriodMaximum
Namespace Prefix: | fsp_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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