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Organization (Details) | 12 Months Ended
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Dec. 31, 2021 entity property
Organization |
Number of REITs in which the entity holds non-controlling common stock interest | 2
Number of Sponsored REITs | 2
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs | 1
Properties |
Number of properties | property | 24
FSP Investments LLC |
Organization |
Ownership interest (as a percent) | 100.00%
FSP Property Management LLC |
Organization |
Ownership interest (as a percent) | 100.00%
FSP Holdings LLC |
Organization |
Ownership interest (as a percent) | 100.00%
FSP Protective TRS Corp. |
Organization |
Ownership interest (as a percent) | 100.00%

X
- Definition Represents the number of promissory notes secured by mortgages on real estate owned by Sponsored REITs. + References No definition available. + Details
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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 common stock interest. + References No definition available. + Details
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Namespace Prefix: | fsp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the number of sponsored REITs the Company held an interest in at period end. + References No definition available. + Details
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Namespace Prefix: | fsp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 272 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055 + Details
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Balance Type: | na
Period Type: | duration

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- Definition The parent entity's interest in net assets of the subsidiary, expressed as a percentage. + References No definition available. + Details
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Balance Type: | na
Period Type: | instant

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- Definition The number of real estate properties owned as of the balance sheet date. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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

X
- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
Name: | srt_OwnershipAxis=fsp_FSPHoldingsLLCMember
Namespace Prefix: |
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Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |