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Mortgage Notes Payable (Narrative) (Details) (USD $)
 12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Sep. 30, 2014
Jul. 01, 2014
Mortgage Notes Payable [Line Items]          
Mortgage notes payable $ 4,309,484,000us-gaap_SecuredDebt $ 4,449,734,000us-gaap_SecuredDebt      
Final maturity date of mortgage notes Apr. 10, 2022        
Gain on forgiveness of debt from discontinued operations 0bxp_DiscontinuedOperationGainLossOnForgivenessOfDebt 20,182,000bxp_DiscontinuedOperationGainLossOnForgivenessOfDebt 0bxp_DiscontinuedOperationGainLossOnForgivenessOfDebt    
Repayments of mortgage notes payable 87,758,000us-gaap_RepaymentsOfSecuredDebt 80,311,000us-gaap_RepaymentsOfSecuredDebt 253,877,000us-gaap_RepaymentsOfSecuredDebt    
(Losses) gains from early extinguishments of debt (10,633,000)us-gaap_GainsLossesOnExtinguishmentOfDebt 122,000us-gaap_GainsLossesOnExtinguishmentOfDebt (4,453,000)us-gaap_GainsLossesOnExtinguishmentOfDebt    
Number Of Mortgage Loans Accounted For Fair Value 4bxp_NumberOfMortgageLoansAccountedForFairValue 4bxp_NumberOfMortgageLoansAccountedForFairValue      
Outstanding balance of mortgage loans accounted for at fair value 2,200,000,000bxp_MortgageLoansAccountedForAtFairValueUponAssumption 2,184,800,000bxp_MortgageLoansAccountedForAtFairValueUponAssumption      
Interest Expense Decrease Due To Recording Mortgage Loans At Fair Value 52,500,000bxp_NterestExpenseDecreaseDueToRecordingMortgageLoansAtFairValue 34,400,000bxp_NterestExpenseDecreaseDueToRecordingMortgageLoansAtFairValue 7,000,000bxp_NterestExpenseDecreaseDueToRecordingMortgageLoansAtFairValue    
Cumulative Liability Related to the Fair Value of Debt Adjustment 138,700,000bxp_CumulativeLiabilityRelatedToFairValueOfDebtAdjustment 191,200,000bxp_CumulativeLiabilityRelatedToFairValueOfDebtAdjustment      
Outside members' notes payable 180,000,000bxp_Outsidemembersnotespayable 180,000,000bxp_Outsidemembersnotespayable      
Interest Payable 163,532,000us-gaap_InterestPayableCurrentAndNoncurrent 167,523,000us-gaap_InterestPayableCurrentAndNoncurrent      
Minimum [Member]          
Mortgage Notes Payable [Line Items]          
Number Real Estate Assets Used For Mortgage Loan Collateral 1bxp_NumberRealEstateAssetsUsedForMortgageLoanCollateral
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 1bxp_NumberRealEstateAssetsUsedForMortgageLoanCollateral
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		      
Fixed Rate Mortgages [Member]          
Mortgage Notes Payable [Line Items]          
Mortgage notes payable 4,300,000,000us-gaap_SecuredDebt
/ us-gaap_LongtermDebtTypeAxis
= bxp_FixedRateMortgagesMember		 4,400,000,000us-gaap_SecuredDebt
/ us-gaap_LongtermDebtTypeAxis
= bxp_FixedRateMortgagesMember		      
Contractual interest rate range, minimum 4.75%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum
/ us-gaap_LongtermDebtTypeAxis
= bxp_FixedRateMortgagesMember		 4.75%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum
/ us-gaap_LongtermDebtTypeAxis
= bxp_FixedRateMortgagesMember		      
Contractual interest rate range, maximum 7.69%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum
/ us-gaap_LongtermDebtTypeAxis
= bxp_FixedRateMortgagesMember		 9.93%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum
/ us-gaap_LongtermDebtTypeAxis
= bxp_FixedRateMortgagesMember		      
Contractual interest rate, weighted vverage interest rate 5.70%us-gaap_DebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= bxp_FixedRateMortgagesMember		 5.70%us-gaap_DebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= bxp_FixedRateMortgagesMember		      
Variable Rate Mortgages [Member]          
Mortgage Notes Payable [Line Items]          
Number of variable rate mortgage loans 0bxp_NumberOfVariableRateMortgageLoans
/ us-gaap_LongtermDebtTypeAxis
= bxp_VariableRateMortgagesMember		 0bxp_NumberOfVariableRateMortgageLoans
/ us-gaap_LongtermDebtTypeAxis
= bxp_VariableRateMortgagesMember		      
London Interbank Offered Rate 0.17%bxp_LondonInterbankOfferedRate
/ us-gaap_LongtermDebtTypeAxis
= bxp_VariableRateMortgagesMember		 0.17%bxp_LondonInterbankOfferedRate
/ us-gaap_LongtermDebtTypeAxis
= bxp_VariableRateMortgagesMember		      
Mortgages [Member] | New Dominion Technology Park, Bldg. Two [Member]          
Mortgage Notes Payable [Line Items]          
Repayments of mortgage notes payable       63,000,000us-gaap_RepaymentsOfSecuredDebt
/ us-gaap_DebtInstrumentAxis
= bxp_NewDominionTechnologyParkBldgTwoMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_MortgagesMember		  
Interest at a fixed rate         5.55%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= bxp_NewDominionTechnologyParkBldgTwoMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_MortgagesMember
Penalty for early repayment of debt       $ 0.00bxp_PenaltyForEarlyRepaymentOfDebt
/ us-gaap_DebtInstrumentAxis
= bxp_NewDominionTechnologyParkBldgTwoMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_MortgagesMember