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Namespace Prefix:	fsp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Represents the useful life of a major finite-lived intangible liability class.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of direct acquisitions of properties with leases.

+ References

No definition available.

+ Details

Name:	fsp_NumberOfDirectAcquisitionsOfPropertiesWithLeases
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of amortization of acquired leases acquired as part of a real property acquisition at below market lease rate with a finite life.

+ References

No definition available.

+ Details

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

- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized during the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized during the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized during the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized during the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized during the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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

- Details

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

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Name:	fsp_AcquiredFiniteLivedIntangibleLiabilitiesByMajorClassAxis=fsp_LeasesAcquiredBelowMarketAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Significant Accounting Policies (Details 4) (USD $)	12 Months Ended
Significant Accounting Policies (Details 4) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Acquired Unfavorable Real Estate Leases and Amortization
Number of acquisitions of properties with leases		3	2
Acquired unfavorable real estate leases
Acquired Unfavorable Real Estate Leases and Amortization
Amortization	$ 3,229,000	$ 3,073,000	$ 1,548,000
Estimated annual amortization for succeeding five years
2015	2,947,000
2016	2,386,000
2017	1,816,000
2018	1,397,000
2019	854,000
2020 and thereafter	$ 1,507,000
Acquired unfavorable real estate leases \| Minimum
Acquired Unfavorable Real Estate Leases and Amortization
Term of lease	10 months
Acquired unfavorable real estate leases \| Maximum
Acquired Unfavorable Real Estate Leases and Amortization
Term of lease	281 months