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Related Party Transactions and Investments in Non-Consolidated Entities - Investment in Sponsored REITs - Property Sold (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) entity	Dec. 31, 2016 USD ($) entity
Investment in Sponsored REITs
Number of properties sold | entity	1	2
FSP 1441 Main Street Corp.
Investment in Sponsored REITs
Repayment of principal	$ 9,000,000
FSP 385 Interlocken Development, Corp.
Investment in Sponsored REITs
Repayment of principal		$ 37,500,000

X

- Definition

Represents the number of properties owned by sponsored REITs which were sold during the period.

+ References

No definition available.

+ Details

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X

- Definition

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

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No definition available.

+ Details

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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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X

- Details

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