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v3.20.1

Related Party Transactions and Investments in Non-Consolidated Entities - Management fees and interest income from loans (Details) - USD ($) | 3 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019
Related Party Transactions and Investments in Non-Consolidated Entities |  |
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 |
Revenue | $ 21,000 | $ 58,000
Revenue, Product and Service [Extensible List] | us-gaap:AssetManagement1Member | us-gaap:AssetManagement1Member

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

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

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- Definition Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Indicates type of revenue from product and service. Includes, but is not limited to, revenue from contract with customer and other sources. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | srt-types:extensibleListItemType
Balance Type: | na
Period Type: | duration