Amount of assets held-for-sale that are not part of a disposal group.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 15
-Paragraph 4
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=6390208&loc=d3e400-110220

+ Details

Name:	us-gaap_AssetsHeldForSaleNotPartOfDisposalGroup
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the amount of gain (loss) on sale of properties during the reporting period net of the applicable income taxes realized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 974
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-15.(a)(1)(ii))
-URI http://asc.fasb.org/extlink&oid=6875705&loc=d3e641549-123027

+ Details

Name:	us-gaap_GainLossOnSaleOfPropertiesNetOfApplicableIncomeTaxes
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

The cash inflow from the sale of formerly productive land held for sale, anything permanently fixed to it, including buildings, structures on it, and so forth.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromSaleOfPropertyHeldForSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=fsp_OfficePropertyInMilpitasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DisposalGroupClassificationAxis=us-gaap_DisposalGroupHeldForSaleOrDisposedOfBySaleNotDiscontinuedOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=fsp_OfficePropertyInMarylandHeightsMissouriMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=fsp_OfficePropertyInFederalWayWashingtonMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Dispositions of Properties (Details) - USD ($) $ in Thousands				3 Months Ended
	Jan. 06, 2017	Dec. 16, 2016	Apr. 05, 2016	Mar. 31, 2017	Dec. 31, 2016
Gain (loss) on sale of property
Gain (loss) on sale of property				$ 2,289
Asset held for sale					$ 3,871
Office Property In Milpitas, California \| Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Gain (loss) on sale of property
Gain (loss) on sale of property	$ 2,300
Office Property In Maryland Heights Missouri \| Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Gain (loss) on sale of property
Gain (loss) on sale of property			$ 4,200
Office Property In Federal Way Washington \| Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Gain (loss) on sale of property
Gain (loss) on sale of property		$ (7,100)
Net Proceeds		$ 7,300