Represents the extension fee paid by the borrower.

No definition available.

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

- Definition

Represents the period from the maturity date of the debt for which the debt facility can be extended.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage of draw fee of each new advance given.

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage of fixed rate of interest on the loans receivable.

+ References

No definition available.

+ Details

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

- 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

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://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.1)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 9

+ Details

Name:	us-gaap_InterestAndFeeIncomeLoansCommercial
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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://www.xbrl.org/2003/role/presentationRef

-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://www.xbrl.org/2003/role/presentationRef

-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=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph e

-Article 9

+ Details

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

- Definition

The percentage points (also referred to as 'margin') added to the reference rate as stated in the loan agreement and used to compute the variable rate on the loans receivable. For instance, the loan agreement might state that the interest rate the borrower is obligated to pay is the London Interbank Offered Rate (LIBOR) plus three quarters (3/4) of a percentage point adjusted quarterly (each three months). LIBOR in this example is the index or reference rate, 3/4 percentage point is the margin, the reference rate plus the margin is the fully indexed rate. This element would be used to report separately the margin.

+ References

No definition available.

+ Details

Name:	us-gaap_LoansReceivableBasisSpreadOnVariableRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The reference rate for the variable rate of the loans receivable, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_MortgageLoansOnRealEstateLoanTypeAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Related Party Transactions and Investments in Non-Consolidated Entities (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Sponsored REITs
Maximum amount of loan	$ 111,300,000
Amount Drawn	93,641,000	99,746,000
Interest income and fees from the Sponsored REIT Loans	5,296,000	5,568,000	9,798,000
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
Secured revolving lines of credit \| FSP Satellite Place Corp.
Sponsored REITs
Maximum amount of loan	5,500,000
Amount Drawn	5,500,000
Sponsored REIT loans, base rate	30-day LIBOR
Sponsored REIT loans, base rate margin (as a percent)	4.40%
Draw Fee (as a percent)	0.50%
Interest rate (as a percent)	4.55%
Secured revolving lines of credit \| FSP 1441 Main Street Corp.
Sponsored REITs
Maximum amount of loan	10,800,000
Amount Drawn	9,000,000
Sponsored REIT loans, base rate	30-day LIBOR
Sponsored REIT loans, base rate margin (as a percent)	4.40%
Draw Fee (as a percent)	0.50%
Interest rate (as a percent)	4.55%
Secured revolving lines of credit \| FSP Energy Tower I Corp.
Sponsored REITs
Maximum amount of loan	20,000,000
Amount Drawn	8,600,000
Sponsored REIT loans, base rate	30-day LIBOR
Sponsored REIT loans, base rate margin (as a percent)	5.00%
Draw Fee (as a percent)	0.50%
Interest rate (as a percent)	5.15%
Secured construction loan \| FSP 385 Interlocken Development Corp.
Sponsored REITs
Maximum amount of loan	42,000,000
Amount Drawn	37,541,000
Sponsored REIT loans, base rate	30-day LIBOR
Sponsored REIT loans, base rate margin (as a percent)	4.40%
Interest rate (as a percent)	4.55%
Mortgage loan secured by property \| FSP Energy Tower I Corp.
Sponsored REITs
Maximum amount of loan	33,000,000
Amount Drawn	33,000,000
Sponsored REIT loans, base rate	30-day LIBOR
Fixed rate of interest (as a percent)	6.41%
Interest rate (as a percent)	6.41%
Extension available on debt	1 year
Extension fee	$ 108,900