No definition available.

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- Definition

Represents the useful life of a major finite-lived intangible liability class.

+ References

No definition available.

+ Details

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Namespace Prefix:	fsp_
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- Definition

Represents the number of direct acquisitions of properties with leases.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
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- Definition

Amount of amortization of acquired leases acquired as part of a real property acquisition at below market lease rate with a finite life.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized during the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized during the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized during the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized during the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of amortization income for leases with a finite life acquired as part of a real property acquisition at below market lease rate expected to be recognized during the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
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Period Type:	duration

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Significant Accounting Policies - Acquired Unfavorable Real Estate Leases and Amortization (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item
Acquired Unfavorable Real Estate Leases and Amortization
Number of acquisitions of properties with leases \| item		3	1
Acquired unfavorable real estate leases
Acquired Unfavorable Real Estate Leases and Amortization
Amortization	$ 3,117,000	$ 3,292,000	$ 3,242,000
Estimated annual amortization for succeeding five years
2018	2,039,000
2019	1,319,000
2020	950,000
2021	620,000
2022	346,000
2023 and thereafter	$ 532,000
Acquired unfavorable real estate leases \| Minimum
Acquired Unfavorable Real Estate Leases and Amortization
Term of lease	7 months
Acquired unfavorable real estate leases \| Maximum
Acquired Unfavorable Real Estate Leases and Amortization
Term of lease	176 months