Reflects the maximum amount of loan due from related parties at the balance sheet date.

No definition available.

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

- Definition

Represents the minimum number of business days in which advance notice has to be provided by the entity to the seller in order to accelerate the closing date of the purchased property.

+ References

No definition available.

+ Details

Name:	fsp_MinimumNumberOfBusinessDaysInWhichAdvanceNoticeHasToBeProvided
Namespace Prefix:	fsp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the aggregate purchase price excluding closing costs and adjustments of real estate properties acquired by the entity during the period.

+ References

No definition available.

+ Details

Name:	fsp_RealEstateAggregatePurchasePrice
Namespace Prefix:	fsp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
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://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-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=6959260&loc=d3e187085-122770

+ Details

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

- Details

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

Subsequent Events (Details) (USD $)	3 Months Ended			0 Months Ended		0 Months Ended
Subsequent Events (Details) (USD $)	Mar. 31, 2013 sqft	Mar. 31, 2012 sqft	Mar. 31, 2013 FSP 385 Interlocken Development Corp. Secured construction loan	Apr. 03, 2013 Subsequent event Office property in Denver, Colorado sqft	Apr. 26, 2013 Subsequent event FSP 385 Interlocken Development Corp. Secured construction loan	Apr. 12, 2013 Cash distribution declared Subsequent event
Subsequent events
Area of property (in square feet)	7,856,859	7,052,068		680,277
Purchase price of property acquired in Denver, Colorado				$ 183,000,000
Minimum number of business days in which advance notice has to be provided				7 days
Cash dividend declared per share (in dollars per share)	$ 0.19	$ 0.19				$ 0.19
Maximum amount of loan	$ 118,800,000		$ 42,000,000		$ 42,000,000