Significant Accounting Policies (Details 5) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Discontinued Operations
Time period within which sale or disposition of properties held for sale is probable	1 year
Tenant Rent Receivables and Straight-line Rent Receivable
Period within which tenant rent receivables are expected to be collected	1 year
Straight-line rent receivable	$ 47,021,000	$ 42,261,000
Allowance for doubtful accounts - Tenant rent receivables
Allowance for doubtful accounts activity
Receivables written-off	119,000	1,237,000	20,000
Increase (decrease) in allowance	394,000	(13,000)	85,000
Allowance for doubtful accounts - Straight-line rent receivable
Allowance for doubtful accounts activity
Receivables written-off		48,000	28,000
Increase (decrease) in allowance	$ 27,000	$ 48,000	$ 28,000

Represents deferred rent receivables net including amounts for assets held for sale.

No definition available.

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

- Definition

Represents the maximum time period within which the sale or disposal of disposal group is expected.

+ References

No definition available.

+ Details

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Namespace Prefix:	fsp_
Data Type:	xbrli:durationItemType
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Period Type:	duration

- Definition

Represents the maximum period within which receivables are expected to be collected.

+ References

No definition available.

+ Details

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

- Details

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Data Type:	xbrli:stringItemType
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- Details

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- Definition

Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs, charged to costs and expenses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 09

-Article 12

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Total of the deductions in a given period to allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs, representing receivables written off as uncollectible and portions of the reserves utilized, respectively.

+ References

+ Details

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- Details

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Namespace Prefix:
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- Details

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