Expected sales price of a property.

No definition available.

Name:	fsp_ExpectedSalesPriceOfProperty
Namespace Prefix:	fsp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the number of office properties sold.

+ References

No definition available.

+ Details

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

- Definition

The cash outflow for the costs related to the sale of property held for sale.

+ References

No definition available.

+ Details

Name:	fsp_PaymentForCostsRelatedToSalePropertyHeldForSale
Namespace Prefix:	fsp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
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-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482130/360-10-45-4

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 360
-SubTopic 10
-Name Accounting Standards Codification
-Section 15
-Paragraph 4
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482309/360-10-15-4

+ Details

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

- Definition

This element represents costs incurred by the lessor that are (a) costs to originate a lease incurred in transactions with independent third parties that (i) result directly from and are essential to acquire that lease and (ii) would not have been incurred had that leasing transaction not occurred and (b) certain costs directly related to specified activities performed by the lessor for that lease. Those activities are: evaluating the prospective lessee's financial condition; evaluating and recording guarantees, collateral, and other security arrangements; negotiating lease terms; preparing and processing lease documents; and closing the transaction. This element is net of accumulated amortization.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 840
-SubTopic 20
-Name Accounting Standards Codification
-Section 25
-Paragraph 16
-Publisher FASB
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-Topic 840
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-Topic 842
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-Name Accounting Standards Codification
-Section 25
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481178/840-20-25-17

+ Details

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

- Definition

Amount of excess of rental income recognized over rental payment required by lease.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 840
-SubTopic 20
-Name Accounting Standards Codification
-Section 35
-Paragraph 2
-Publisher FASB
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-Topic 840
-SubTopic 20
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-Paragraph 1
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-Topic 842
-SubTopic 30
-Name Accounting Standards Codification
-Section 25
-Paragraph 11
-Subparagraph (a)
-Publisher FASB
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-Name Accounting Standards Codification
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-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481178/840-20-25-2

+ Details

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

- Definition

The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-28

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-Topic 360
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482130/360-10-45-5

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-Topic 360
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482099/360-10-50-3

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems
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://fasb.org/us-gaap/role/ref/legacyRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 12
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-12

+ Details

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

- Definition

Amount of real estate investment property, net of accumulated depreciation, which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1)(d))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1

+ Details

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

- Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Dispositions of Properties and Asset Held for Sale (Details)			3 Months Ended
	Jan. 26, 2024 USD ($) property	Mar. 10, 2023 USD ($) property	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Dispositions of Property
Asset held for sale			$ 38,947,000		$ 73,318,000
Real estate assets			884,733,000		890,824,000
Straight-line rent receivable			40,357,000		40,397,000
Deferred leasing commissions			24,730,000		23,664,000
Property in Atlanta, Georgia
Dispositions of Property
Asset held for sale			39,000,000.0	$ 39,000,000.0	39,000,000.0
Gain (loss) on sale of property				(20,500,000)
Expected sales price				40,000,000.0
Real estate assets			52,200,000		52,200,000
Straight-line rent receivable			4,400,000		4,400,000
Deferred leasing commissions			2,900,000		$ 2,900,000
Impairment charge				$ 20,500,000
Office Property in Elk Grove Village, Illinois
Dispositions of Property
Number of properties sold \| property		1
Gain (loss) on sale of property		$ 8,400,000
Sale price		$ 29,100,000
Properties In Richardson, Texas
Dispositions of Property
Number of properties sold \| property	1
Gain (loss) on sale of property	$ (2,100,000)
Sale price	$ 35,000,000.0
Additional costs related to sale			$ 5,000