Related Party Transactions and Investments in Non-Consolidated Entities (Details 2) (USD $)	9 Months Ended
Sep. 30, 2011 sqft Segment Y Property D Entity	Sep. 30, 2010	Dec. 31, 2010
Balance Sheet Data (unaudited):
Real estate, net	$ 758,425,000		$ 767,938,000
Other assets	137,818,000		105,218,000
Total Liabilities	(292,460,000)		(256,612,000)
Total stockholders' equity	603,783,000		616,544,000
Operating Data (unaudited):
Rental revenues	83,754,000	68,383,000
Other revenues	52,000	140,000
Operating and maintenance expenses	(41,132,000)	(38,206,000)
Selling, general and administrative	(787,000)
Depreciation and amortization	(24,988,000)	(20,464,000)
Interest expense	(12,754,000)	(6,847,000)
Net income	4,145,000	3,006,000
Syndication, development and transaction fees from non-consolidated entities	7,869,000	1,827,000
Asset management fees, low end of range (as a percent)	1.00%
Asset management fees, high end of range (as a percent)	5.00%
Notice period for cancellation of applicable contracts (in days)	30
Asset management fee income from non-consolidated entities	$ 715,000	$ 599,000

High end of the range of the asset management fee as a percentage of collected rents.

Name:	fsp_AssetManagementFeesPercentageHighEndOfRange
Namespace Prefix:	fsp
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Low end of the range of the asset management fee as a percentage of collected rents.

+ Details

Name:	fsp_AssetManagementFeesPercentageLowEndOfRange
Namespace Prefix:	fsp
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of depreciation and amortization reported by an equity method investment of the entity.

+ Details

Name:	fsp_EquityMethodInvestmentSummarizedFinancialInformationDepreciationAndAmortization
Namespace Prefix:	fsp
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of Interest expense reported by an equity method investment of the entity.

+ Details

Name:	fsp_EquityMethodInvestmentSummarizedFinancialInformationInterestExpense
Namespace Prefix:	fsp
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of operating and maintenance expense reported by an equity method investment of the entity.

+ Details

Name:	fsp_EquityMethodInvestmentSummarizedFinancialInformationOperatingAndMaintenanceExpense
Namespace Prefix:	fsp
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of other assets reported by an equity method investment of the entity.

+ Details

Name:	fsp_EquityMethodInvestmentSummarizedFinancialInformationOtherAssets
Namespace Prefix:	fsp
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of other revenue reported by an equity method investment of the entity.

+ Details

Name:	fsp_EquityMethodInvestmentSummarizedFinancialInformationOtherRevenue
Namespace Prefix:	fsp
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of real estate, net assets reported by an equity method investment of the entity.

+ Details

Name:	fsp_EquityMethodInvestmentSummarizedFinancialInformationRealEstateNet
Namespace Prefix:	fsp
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of rental revenue reported by an equity method investment of the entity.

+ Details

Name:	fsp_EquityMethodInvestmentSummarizedFinancialInformationRentalRevenue
Namespace Prefix:	fsp
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of operating and maintenance expense reported by an equity method investment of the entity.

+ Details

Name:	fsp_EquityMethodInvestmentSummarizedFinancialInformationSellingGeneralAndAdministrativeExpense
Namespace Prefix:	fsp
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the notice period for cancellation of applicable contracts.

+ Details

Name:	fsp_NoticePeriodForCancellationOfAssetManagementFeeContract
Namespace Prefix:	fsp
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Syndication fees from the sale of securities in sponsored REITs are generally recognized upon an investor closing. Transaction fees relating to the loan commitment fees and acquisition fees are recognized upon an investor closing and the subsequent payment of the sponsored REIT's loan. Development fees are recognized upon an investor closing and once the service has been provided. Fees related to organizational, offering and other expenditures are recognized upon the final investor closing of the sponsored REIT.

+ Details

Name:	fsp_SyndicationDevelopmentAndTransactionFeesRevenue
Namespace Prefix:	fsp
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Revenue recognized in the period for (1) performance (incentive) fees based on the investment results achieved for management of certain institutional accounts and hedge funds, (2) schedule-based fees earned for management of mutual funds and closed-end funds, based either on average daily net assets or on a combination of the average daily net assets and gross income, and (3) other investment management and advisory fees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Chapter 4

-Paragraph 9, 51, 59

-IssueDate 2006-05-01

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(a),(d))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

Name:	us-gaap_AssetManagementFees
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssetsAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of equity (capital), including the noncontrolling interest (if any), reported by an equity method investment of the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 159

-Paragraph 18

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 18

-Paragraph 20

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Name Accounting Research Bulletin (ARB)

-Publisher AICPA

-Number 51

-Paragraph 26

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquity
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_EquityMethodInvestmentSummarizedFinancialInformationIncomeStatementAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of liabilities reported by an equity method investment of the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(g))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

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

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 18

-Paragraph 20

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph bb

-Article 1

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33912-111571

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 159

-Paragraph 18

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 4

-Section 08

-Paragraph g

-Subparagraph 1, 2

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph w

-Article 1

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 28

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612

+ Details

Name:	us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilities
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of net income (loss) reported by an equity method investment of the entity.

+ References

+ Details

Name:	us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration