Significant Accounting Policies (Details 5) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Discontinued Operations
Time period within which sale or disposition of properties held for sale is probable	1 year
Cash and Cash Equivalents
Maximum term of original maturity to classify instruments as cash equivalents	3 months
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	$ 35,441,000	$ 28,545,000
Allowance for doubtful accounts - Tenant rent receivables
Allowance for doubtful accounts activity
Receivables charged off	20,000	399,000	195,000
Increase (decrease) in allowance	85,000	34,000	1,175,000
Allowance for doubtful accounts - Straight-line rent receivable
Allowance for doubtful accounts activity
Receivables charged off	28,000	567,000
Increase (decrease) in allowance	$ 28,000	$ 2,000	$ 600,000

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

No definition available.

Name:	fsp_DeferredRentReceivableNetIncludingAmountsForAssetsHeldForSale
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

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

- Definition

Represents the maximum original term to maturity for an instrument to be classified as cash equivalents.

+ References

No definition available.

+ Details

Name:	fsp_MaximumTermOfOriginalMaturityToClassifyInstrumentsAsCashEquivalents
Namespace Prefix:	fsp_
Data Type:	xbrli:durationItemType
Balance Type:	na
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

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

- Details

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

- Details

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

- 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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 09

-Article 12

Reference 2: 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=6881521&loc=d3e24092-122690

+ Details

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

- Details

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