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Significant Accounting Policies - Basis, Estimates and Inv in non-consolidated REITs (Detail)	Dec. 31, 2018 USD ($) entity	Dec. 31, 2017 entity	Dec. 31, 2016 entity
Significant Accounting Policies
Number of corporations organized to operate as real estate investment trusts (REITs)	3	6	7
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

X

- Definition

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.

+ References

No definition available.

+ Details

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

X

- Definition

Represents the number of entities in which the reporting entity has a non-controlling common stock interest.

+ References

No definition available.

+ Details

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Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents the number of entities in which the reporting entity has a non-controlling preferred stock interest.

+ References

No definition available.

+ Details

Name:	fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingPreferredStockInterest
Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration