Related Party Transactions and Investments in Non-Consolidated Entities - Investment in Sponsored REITs - Property Sold (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
FSP Energy Tower I Corp.
Investment in Sponsored REITs
Repayment of principal	$ 51.0

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585

Name:	us-gaap_RepaymentsOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration