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Organization (Details)	12 Months Ended
	Dec. 31, 2022 entity property	Dec. 31, 2021 entity	Dec. 31, 2020 entity
Organization
Number of REITs in which the entity holds non-controlling common stock interest	1	2	2
Number of Sponsored REITs	1
Number of promissory notes secured by mortgages on real estate owned by Sponsored REITs	1
Properties
Number of properties | property	21
FSP Investments LLC
Organization
Ownership interest (as a percent)	100.00%
FSP Property Management LLC
Organization
Ownership interest (as a percent)	100.00%
FSP Holdings LLC
Organization
Ownership interest (as a percent)	100.00%
FSP Protective TRS Corp.
Organization
Ownership interest (as a percent)	100.00%

X

- Definition

Represents the number of promissory notes secured by mortgages on real estate owned by Sponsored REITs.

+ References

No definition available.

+ Details

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Balance Type:	na
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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Balance Type:	na
Period Type:	instant

X

- Definition

Represents the number of sponsored REITs the Company held an interest in at period end.

+ References

No definition available.

+ Details

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Namespace Prefix:	fsp_
Data Type:	xbrli:integerItemType
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- References

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- Definition

Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 272
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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- References

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+ Details

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- Details

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X

- Details

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X

- Details

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X

- Details

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