Represents the number of office properties to be sold.

No definition available.

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

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow from the sale of formerly productive land held for sale, anything permanently fixed to it, including buildings, structures on it, and so forth.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3179-108585

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
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Period Type:

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fsp_OfficePropertiesInChantillyVaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fsp_OfficePropertiesInChesterfieldMoMember
Namespace Prefix:
Data Type:	na
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Subsequent Events (Details) $ / shares in Units, $ in Millions				3 Months Ended
	Jul. 13, 2021 USD ($) property	Jul. 09, 2021 USD ($) property	Jul. 06, 2021 $ / shares	Jun. 30, 2021 ft² $ / shares	Mar. 31, 2021 $ / shares	Jun. 30, 2020 ft² $ / shares	Mar. 31, 2020 $ / shares
Subsequent Events
Cash dividend declared per share (in dollars per share) \| $ / shares				$ 0.09	$ 0.09	$ 0.09	$ 0.09
Area of Real Estate Property \| ft²				8,228,148		9,508,226
Cash distribution declared \| Subsequent Events.
Subsequent Events
Cash dividend declared per share (in dollars per share) \| $ / shares			$ 0.09
Office properties in Chantilly, VA \| Subsequent Events.
Subsequent Events
Number of office properties to be sold \| property		2
Sale price \| $		$ 40
Office properties in Chesterfield, MO \| Subsequent Events.
Subsequent Events
Number of office properties to be sold \| property	2
Sale price \| $	$ 67