Represents the number of promissory notes secured by mortgages on real estate owned by Sponsored REITs.

No definition available.

Name:	fsp_NumberOfPromissoryNotesSecuredByMortgagesOnRealEstateOwnedBySponsoredREITs
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of properties in redevelopment.

+ References

No definition available.

+ Details

Name:	fsp_NumberOfPropertiesInRedevelopment
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- 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

- Definition

Represents the number of sponsored REITs the Company held an interest in at period end.

+ References

No definition available.

+ Details

Name:	fsp_NumberOfSponsoredReitsCompanyHeldInterestIn
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

Name:	fsp_OrganizationAndConsolidationLineItems
Namespace Prefix:	fsp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPMembersOrLimitedPartnersOwnershipInterest
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfRealEstateProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_RealEstateInvestmentsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_OwnershipAxis=fsp_FSPInvestmentsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=fsp_FSPPropertyManagementLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=fsp_FSPHoldingsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=fsp_FSPProtectiveTRSCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Organization (Details)	12 Months Ended
Organization (Details)	Dec. 31, 2020 property entity	Dec. 31, 2019 entity	Dec. 31, 2018 entity
Organization
Number of REITs in which the entity holds non-controlling common stock interest	2	2	3
Number of Sponsored REITs	2		3
Number of properties in redevelopment \| property	2
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	1
Properties
Number of properties \| property	32
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%