Significant Accounting Policies - Rent Receivables (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Tenant Rent Receivables and Straight-line Rent Receivable
Period within which tenant rent receivables are expected to be collected	1 year
Straight-line rent receivable	$ 67.8	$ 66.9

Represents deferred rent receivables net including amounts for assets held for sale.

No definition available.

Name:	fsp_DeferredRentReceivableNetIncludingAmountsForAssetsHeldForSale
Namespace Prefix:	fsp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant