Significant Accounting Policies - Basis, Estimates and Inv in non-consolidated REITs (Detail)	Dec. 31, 2017 USD ($) entity	Dec. 31, 2016 entity	Dec. 31, 2015 entity
Significant Accounting Policies
Number of corporations organized to operate as real estate investment trusts (REITs)	6	7	9
Number of REITs in which the entity holds non-controlling preferred stock interest	2
Required investment balance for recognition of distributions received as income \| $	$ 0

Represents the amount of required balance of investments for recognition of distributions received as income. These investments are accounted for under the equity method of accounting.

No definition available.

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