Significant Accounting Policies - Asset Held For Sale, Cash (Details)	12 Months Ended
Dec. 31, 2017
Discontinued Operations
Time period within which sale or disposition of properties held for sale is probable	1 year

Represents the maximum time period within which the sale or disposal of disposal group is expected.

No definition available.

Name:	fsp_DisposalGroupIncludingDiscontinuedOperationMaximumPeriodForDisposal
Namespace Prefix:	fsp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration