Significant Accounting Policies - Rent Receivables (Details) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 53,194,000	$ 50,930,000
Allowance for doubtful accounts - Tenant rent receivables
Tenant Rent Receivables and Straight-line Rent Receivable
Increase (decrease) in allowance	178,000	78,000	$ (106,000)
Receivables written-off	$ 28,000	$ 108,000	89,000
Allowance for doubtful accounts - Straight-line rent receivable
Tenant Rent Receivables and Straight-line Rent Receivable
Increase (decrease) in allowance			(112,000)
Receivables written-off			$ 112,000

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 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

- References

No definition available.

+ Details

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

- Definition

Amount of increase in the allowances and reserves, which consist of the valuation and qualifying accounts that are either netted against the cost of an asset or that reflect a liability established to represent expected future costs, from charges 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
-Subparagraph (SX 210.12-09)
-URI http://asc.fasb.org/extlink&oid=26873400&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_ValuationAllowancesAndReservesTypeAxis=fsp_AllowanceForTenantRentReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowancesAndReservesTypeAxis=fsp_AllowanceForStraightLineRentReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: