Related Party Transactions and Investments in Non-Consolidated Entities - Investment in Sponsored REITs (Details) - entity	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transactions and Investments in Non-Consolidated Entities
Number of REITs in which the entity holds non-controlling common stock interest	2	2	3
Number of REITs in which the entity holds non-controlling preferred stock interest	2	2

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

No definition available.

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