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Significant Accounting Policies - Basis, Estimates and Inv in non-consolidated REITs (Detail) | Dec. 31, 2017 USD ($) entity | Dec. 31, 2016 entity | Dec. 31, 2015 entity
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Significant Accounting Policies |  |  |
Number of corporations organized to operate as real estate investment trusts (REITs) | 6 | 7 | 9
Number of REITs in which the entity holds non-controlling preferred stock interest | 2 |  |
Required investment balance for recognition of distributions received as income | $ | $ 0 |  |

X
- Definition Represents the amount of required balance of investments for recognition of distributions received as income. These investments are accounted for under the equity method of accounting. + References No definition available. + Details
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Namespace Prefix: | fsp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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
Name: | fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingCommonStockInterest
Namespace Prefix: | fsp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the number of entities in which the reporting entity has a non-controlling preferred stock interest. + References No definition available. + Details
Name: | fsp_NumberOfREITsInWhichEntityHoldsNoncontrollingPreferredStockInterest
Namespace Prefix: | fsp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration