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Bank Note Payable and Term Note Payable (Details) (USD $) | 12 Months Ended | 0 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2014 | Sep. 27, 2012 | Oct. 29, 2014 | Aug. 26, 2013
Bank note payable and term note payable |  |  |  |  |  |
Amount drawn down | $ 220,000,000 us-gaap_ProceedsFromIssuanceOfUnsecuredDebt | $ 400,000,000 us-gaap_ProceedsFromIssuanceOfUnsecuredDebt |  |  |  |
Borrowings outstanding | 306,500,000 us-gaap_LineOfCredit |  | 268,000,000 us-gaap_LineOfCredit |  |  |
BAML Revolver |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Total available |  |  | 500,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |  |
Additional borrowing capacity allowed by exercising an accordion feature |  |  | 250,000,000 fsp_LineOfCreditFacilityMaximumBorrowingCapacityIncreaseAfterAccordionFeature / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |  |
Borrowings outstanding | 306,500,000 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  | 268,000,000 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |  |
Extension available on debt |  |  | 12 months |  |  |
Facility fee (as a percent) |  |  | 0.25%us-gaap_LineOfCreditFacilityCommitmentFeePercentage / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |  |
Weighted average interest rate (as a percent) | 1.82%us-gaap_DebtWeightedAverageInterestRate / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  | 1.41%us-gaap_DebtWeightedAverageInterestRate / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |  |
Interest rate during period (as a percent) |  |  | 1.60%us-gaap_LineOfCreditFacilityInterestRateDuringPeriod / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |  |
BAML Revolver | Baa1 | BBB+ |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Facility fee (as a percent) |  |  | 0.15%us-gaap_LineOfCreditFacilityCommitmentFeePercentage / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember |  |  |
BAML Revolver | Baa2 | BBB |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Facility fee (as a percent) |  |  | 0.20%us-gaap_LineOfCreditFacilityCommitmentFeePercentage / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa2RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember |  |  |
BAML Revolver | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Facility fee (as a percent) |  |  | 0.25%us-gaap_LineOfCreditFacilityCommitmentFeePercentage / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember |  |  |
BAML Revolver | Minimum | A3 | A- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Facility fee (as a percent) |  |  | 0.125%us-gaap_LineOfCreditFacilityCommitmentFeePercentage / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / us-gaap_RangeAxis = us-gaap_MinimumMember |  |  |
BAML Revolver | Maximum | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Facility fee (as a percent) |  |  | 0.30%us-gaap_LineOfCreditFacilityCommitmentFeePercentage / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |
BAML Term Loan |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Principal amount of loan |  |  | 400,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember |  |  |
Amount drawn down |  |  |  | 400,000,000 us-gaap_ProceedsFromIssuanceOfUnsecuredDebt / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember |  |
Weighted average interest rate (as a percent) |  |  | 2.20%us-gaap_DebtWeightedAverageInterestRate / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember |  |  |
LIBOR | BAML Revolver |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
LIBOR | BAML Revolver | Baa1 | BBB+ |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.925%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
LIBOR | BAML Revolver | Baa2 | BBB |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.05%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa2RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
LIBOR | BAML Revolver | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
LIBOR | BAML Revolver | Minimum | A3 | A- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.875%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
LIBOR | BAML Revolver | Maximum | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.65%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
LIBOR | BAML Term Loan |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.45%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
Fixed rate (as a percent) |  |  | 0.75%us-gaap_DerivativeFixedInterestRate / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
Term pursuant to interest rate swap agreement |  |  |  | 5 years |  |
LIBOR | BAML Term Loan | Baa1 | BBB+ |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.025%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
LIBOR | BAML Term Loan | Baa2 | BBB |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.20%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa2RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
LIBOR | BAML Term Loan | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.45%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
LIBOR | BAML Term Loan | Minimum | A3 | A- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.95%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
LIBOR | BAML Term Loan | Maximum | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.90%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
Base Rate | BAML Revolver |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Revolver | Baa1 | BBB+ |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Revolver | Baa2 | BBB |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.05%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa2RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Revolver | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Revolver | Minimum | A3 | A- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Revolver | Maximum | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.65%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Term Loan |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.45%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Term Loan | Baa1 | BBB+ |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.025%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Term Loan | Baa2 | BBB |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.20%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa2RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Term Loan | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.45%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Term Loan | Minimum | A3 | A- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Base Rate | BAML Term Loan | Maximum | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.90%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_UnsecuredDebtMember / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
Federal Funds Rate | BAML Credit Facility |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_VariableRateAxis = fsp_FederalFundsRateMember |  |  |
One month LIBOR | BAML Credit Facility |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_VariableRateAxis = fsp_LIBOROneMonthMember |  |  |
BMO Term Loan |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Principal amount of loan |  |  |  |  |  | 220,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember
Amount drawn down |  |  |  |  |  | 220,000,000 us-gaap_ProceedsFromIssuanceOfUnsecuredDebt / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember
Term of the borrowing |  |  |  |  | 7 years |
Additional loans allowed by exercising an accordion feature |  |  |  |  | $ 50,000,000 fsp_DebtInstrumentMaximumAdditionalLoanAmountUnderAccordionFeature / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember |
Debt instrument overall rate (basis spread plus variable rate), end of period (as a percent) | 4.22%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember |  | 3.97%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember |  |  |
BMO Term Loan | LIBOR |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.65%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
Fixed rate (as a percent) |  |  | 2.32%us-gaap_DerivativeFixedInterestRate / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
Term pursuant to interest rate swap agreement |  |  |  |  |  | 7 years
BMO Term Loan | LIBOR | Baa1 | BBB+ |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.15%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
BMO Term Loan | LIBOR | Baa2 | BBB |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.35%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa2RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
BMO Term Loan | LIBOR | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.65%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
BMO Term Loan | LIBOR | Minimum | A3 | A- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.05%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
BMO Term Loan | LIBOR | Maximum | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 2.15%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |  |
BMO Term Loan | Base Rate |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.65%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
BMO Term Loan | Base Rate | Baa1 | BBB+ |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
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BMO Term Loan | Base Rate | Baa2 | BBB |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.35%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa2RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBRatingMember / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
BMO Term Loan | Base Rate | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.65%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
BMO Term Loan | Base Rate | Minimum | A3 | A- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.05%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
BMO Term Loan | Base Rate | Maximum | Baa3 | BBB- |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.15%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa3RatingMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBMinusRatingMember / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_VariableRateAxis = us-gaap_BaseRateMember |  |  |
BMO Term Loan | Federal Funds Rate |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_VariableRateAxis = fsp_FederalFundsRateMember |  |  |
BMO Term Loan | One month LIBOR |  |  |  |  |  |
Bank note payable and term note payable |  |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = fsp_BMOTermLoanMember / us-gaap_VariableRateAxis = fsp_LIBOROneMonthMember |  |  |

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- Definition Represents the period from the maturity date of the debt for which the debt facility can be extended. + References No definition available. + Details
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- Definition Maximum amount of additional loans that may be issued under the accordian feature of the debt agreement subject to receipt of lender commitments and satisfaction of certain customary conditions. + References No definition available. + Details
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- Definition The increase in the maximum borrowing capacity under the credit facility upon exercise of the accordion feature which is subject to receipt of lender commitments and satisfaction of certain customary conditions. + References No definition available. + Details
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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400 + Details
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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Weighted average interest rate of debt outstanding. + References No definition available. + Details
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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used. + References No definition available. + Details
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- Definition The effective interest rate during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Definition Maximum period the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994 + Details
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- Definition The cash inflow from the issuance of long-term debt that is not secured by collateral. Excludes proceeds from tax exempt unsecured debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
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