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Significant Accounting Policies (Details 7) (USD $)	12 Months Ended
	Dec. 31, 2014 item
Concentration of Credit Risks
Number of banks in which the entity maintains cash balances	2fsp_NumberOfBanksInWhichEntityMaintainsCashBalances
Number of interest rate swap agreements	2us-gaap_DerivativeNumberOfInstrumentsHeld
Cash balances with financial institutions | Credit concentration risk | Minimum
Concentration of Credit Risks
Insurance limit provided by Federal Deposit Insurance Corporation	250,000us-gaap_CashFDICInsuredAmount / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_DemandDepositsMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Annualized rental revenues | Single tenant rental revenues
Concentration of Credit Risks
Percentage of annualized rental revenues required for qualification as major tenant	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = fsp_AnnualizedRentalRevenuesMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember

X

- Definition

Represents the number of banks in which the entity maintains cash balances.

+ References

No definition available.

+ Details

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

X

- Definition

The amount of cash deposited in financial institutions as of the balance sheet date that is insured by the Federal Deposit Insurance Corporation.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_ConcentrationRiskLineItems
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 21

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 18

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

X

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959

+ Details

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Balance Type:	na
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X

- Details

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X

- Details

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Period Type: