Related Party Transactions and Investments in Non-Consolidated Entities - Management fees and interest income from loans (Details) - USD ($)	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Management fees and interest income from loans
Asset management fees, low end of range (as a percent)	1.00%
Asset management fees, high end of range (as a percent)	5.00%
Notice period for cancellation of applicable contracts	30 days
Asset management fee income from non-consolidated entities	$ 459,000	$ 474,000
Sponsored REITs
Management fees and interest income from loans
Impairment of Sponsored REIT	$ 0

High end of the range of the asset management fee as a percentage of collected rents.

No definition available.

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- Definition

Low end of the range of the asset management fee as a percentage of collected rents.

+ References

No definition available.

+ Details

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Namespace Prefix:	fsp_
Data Type:	num:percentItemType
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Period Type:	duration

- Definition

Represents the notice period for cancellation of applicable contracts.

+ References

No definition available.

+ Details

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- Definition

Amount of fixed fee revenue for the management of an investment fund portfolio. Excludes investment advisory, distribution and servicing, and performance fees.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953550-111524

+ Details

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Balance Type:	debit
Period Type:	instant

- Details

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