Significant Accounting Policies - Basis, Estimates and Inv in non-consolidated REITs (Detail) - entity	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Significant Accounting Policies
Number of corporations organized to operate as real estate investment trusts (REITs)	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