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Related Party Transactions and Investments in Non-Consolidated Entities - Investment in Sponsored REITs (Details) - entity	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transactions and Investments in Non-Consolidated Entities
Number of REITs in which the entity holds non-controlling common stock interest	3	6	7
Number of REITs in which the entity holds non-controlling preferred stock interest	2

X

- Definition

Represents the number of entities in which the reporting entity has a non-controlling common stock interest.

+ References

No definition available.

+ Details

Name:	fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingCommonStockInterest
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents the number of entities in which the reporting entity has a non-controlling preferred stock interest.

+ References

No definition available.

+ Details

Name:	fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingPreferredStockInterest
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- References

No definition available.

+ Details

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