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Organization, Properties, Basis of Presentation, Financial Instruments and Recent Accounting Standards (Details 2) (USD $) In Millions, unless otherwise specified			0 Months Ended
	Mar. 31, 2013 property sqft	Mar. 31, 2012 sqft property	Mar. 08, 2013 Office property in Atlanta, Georgia sqft
Commercial real estate:
Number of properties	37	36
Rentable square feet	7,856,859	7,052,068	621,007
Purchase price of property acquired in Atlanta, Georgia			$ 157.9

X

- Definition

Represents the aggregate purchase price excluding closing costs and adjustments of real estate properties acquired by the entity during the period.

+ References

No definition available.

+ Details

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

X

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfRealEstateProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Details

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