Reflects the maximum amount of loan due from related parties at the balance sheet date.

No definition available.

Name:	fsp_LoansAndLeasesReceivableRelatedPartiesMaximumAmountOfLoan
Namespace Prefix:	fsp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Extended term of mortgage loan on real estate, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	fsp_MortgageLoanOnRealEstateExtendedTerm
Namespace Prefix:	fsp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Stated maximum period for maturity of the mortgage loan receivable on real estate.

+ References

No definition available.

+ Details

Name:	fsp_MortgageLoanOnRealEstateLoansMaturityPeriodMaximum
Namespace Prefix:	fsp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Stated minimum period for maturity of the mortgage loan receivable on real estate.

+ References

No definition available.

+ Details

Name:	fsp_MortgageLoanOnRealEstateLoansMaturityPeriodMinimum
Namespace Prefix:	fsp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the exit fee to be paid by the borrowed when repayment is made.

+ References

No definition available.

+ Details

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

- Definition

The interest rate of the mortgage loan receivable at the end of the period.

+ References

No definition available.

+ Details

Name:	fsp_MortgageLoansOnRealEstateInterestRateAtEndOfPeriod
Namespace Prefix:	fsp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Interest and fee income related to commercial loans including industrial and agricultural, real estate (commercial and residential, construction and development), trade financing, and lease financing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.1)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

+ Details

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

- Definition

Expenses paid for obtaining loans which includes expenses such as application and origination fees.

+ References

No definition available.

+ Details

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

- Definition

For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(e))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_LoansAndLeasesReceivableRelatedParties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_RelatedPartyTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fsp_SponsoredREITsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fsp_FspMonumentCircleLlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Party Transactions and Investments in Non-Consolidated Entities - Sponsored REIT Loans outstanding (Details) - USD ($)		12 Months Ended
	Dec. 06, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 05, 2020
Sponsored REITs
Maximum amount of loan		$ 21,000,000
Amount Drawn		21,000,000	$ 21,000,000
Interest income and fees from the Sponsored REIT Loans		1,500,000	3,300,000	$ 4,600,000
Related party mortgage loan receivable		$ 21,000,000	$ 21,000,000
Sponsored REITs
Sponsored REITs
Term of sponsored REIT loan secured by mortgage, minimum		1 year
Term of sponsored REIT loan secured by mortgage, maximum		3 years
Mortgage loan secured by property \| Sponsored REITs
Sponsored REITs
Interest rate (as a percent)	7.51%				7.19%
Mortgage Loan On Real Estate, Extended Term	2 years
Mortgage loan secured by property \| Sponsored REITs \| Fair Value, Inputs, Level 3 [Member]
Sponsored REITs
Amount Drawn		$ 20,400,000
Related party mortgage loan receivable		20,400,000
Mortgage loan secured by property \| FSP Monument Circle LLC
Sponsored REITs
Maximum amount of loan		21,000,000
Amount Drawn		$ 21,000,000
Interest rate (as a percent)		7.51%
Origination fee		$ 164,000
Exit fee		38,000
Related party mortgage loan receivable		$ 21,000,000