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Related Party Transactions and Investments in Non-Consolidated Entities - Management fees and interest income from loans (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Management fees and interest income from loans |  |  |
Asset management fees, low end of range (as a percent) | 1.00% |  |
Asset management fees, high end of range (as a percent) | 5.00% |  |
Notice period for cancellation of applicable contracts | 30 days |  |
Asset management fee income from non-consolidated entities | $ 598,000 | $ 631,000 | $ 701,000

X
- Definition High end of the range of the asset management fee as a percentage of collected rents. + References No definition available. + Details
Name: | fsp_AssetManagementFeesPercentageHighEndOfRange
Namespace Prefix: | fsp_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Low end of the range of the asset management fee as a percentage of collected rents. + References No definition available. + Details
Name: | fsp_AssetManagementFeesPercentageLowEndOfRange
Namespace Prefix: | fsp_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the notice period for cancellation of applicable contracts. + References No definition available. + Details
Name: | fsp_NoticePeriodForCancellationOfAssetManagementFeeContract
Namespace Prefix: | fsp_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of fixed fee revenue for the management of an investment fund portfolio. Excludes investment advisory, distribution and servicing, and performance fees. + References No definition available. + Details
Name: | us-gaap_AssetManagementFees1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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