Related Party Transactions and Investments in Non-Consolidated Entities - Investment in Sponsored REITs (Details) - entity	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transactions and Investments in Non-Consolidated Entities
Number of REITs in which the entity holds non-controlling common stock interest	6	7	9
Number of REITs in which the entity holds non-controlling preferred stock interest	2
Number of properties sold	1	2	1

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