Fixed interest rate related to the interest rate derivative.

No definition available.

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of asset hedged in fair value hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL109999712-113959

+ Details

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

- Definition

Amount of liability hedged in fair value hedging relationship.

+ References

+ Details

Name:	us-gaap_HedgedLiabilityFairValueHedge
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value as of the balance sheet date of all interest rate derivatives designated as cash flow hedging instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=117331979&loc=d3e41228-113958

+ Details

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

- Definition

The amount of net gains or losses on interest rate cash flow hedges reclassified during the period to earnings from accumulated other comprehensive income upon the hedged transaction affecting earnings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5624171-113959

+ Details

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

- Definition

The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994

+ Details

Name:	us-gaap_InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash outflow for a financial contract that meets the hedge criteria as either cash flow hedge, fair value hedge or hedge of net investment in foreign operations.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3291-108585

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 27
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3581-108585

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=fsp_JPMTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fsp_JpmInterestRateSwap2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fsp_BmoInterestRateSwap2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Instruments: Derivatives and Hedging (Details) - USD ($) $ in Thousands		6 Months Ended
	Jun. 04, 2021	Jun. 30, 2022	Dec. 31, 2021
Financial Instruments: Derivatives and Hedging
Interest reclassified from accumulated other comprehensive income into interest expense		$ 1,900
Amount estimated to be reclassified into earnings within next 12 months		1,200
JPM Term Loan
Financial Instruments: Derivatives and Hedging
Hedged amount of portion of the future rate risk		100,000
2019 JPM Interest Rate Swap
Financial Instruments: Derivatives and Hedging
Payment to terminate interest rate swap	$ 1,200
2019 BMO Interest Rate Swap
Financial Instruments: Derivatives and Hedging
Fair value of hedged asset		2,000	$ 2,000
Hedged amount of portion of the future rate risk		5,200	5,200
Payment to terminate interest rate swap	600
2019 BMO Interest Rate Swap \| Cash flow hedges
Financial Instruments: Derivatives and Hedging
Notional Value	$ 165,000	$ 165,000
Strike Rate (as a percent)		2.39%
2019 BMO Interest Rate Swap \| Cash flow hedges \| Level 2
Financial Instruments: Derivatives and Hedging
Fair Value		$ 1,951	$ (5,239)