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 entities in which the reporting entity has a non-controlling preferred stock interest.

+ References

No definition available.

+ Details

Name:	fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingPreferredStockInterest
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://www.xbrl.org/2003/role/presentationRef
-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:	us-gaap_OwnershipAxis=fsp_FSPInvestmentsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_OwnershipAxis=fsp_FSPPropertyManagementLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_OwnershipAxis=fsp_FSPHoldingsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_OwnershipAxis=fsp_FSPProtectiveTRSCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis=fsp_RevolvingLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Organization (Details)	12 Months Ended
Organization (Details)	Dec. 31, 2017 property entity item	Dec. 31, 2016 entity	Dec. 31, 2015 entity
Organization
Number of REITs in which the entity holds non-controlling common stock interest \| entity	6	7	9
Number of REITs in which the entity holds non-controlling preferred stock interest \| entity	2
Number of Sponsored REITs \| entity	6	7	9
Number of properties in redevelopment	1
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	4
Properties
Number of properties \| property	34
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%
Mortgage loan secured by property
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	2
Secured revolving lines of credit
Organization
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	2