Debt (Notes)	6 Months Ended
Jun. 30, 2017
Debt [Abstract]
Debt Disclosure [Text Block]	5. Debt Mortgage Notes Payable, Net, Mezzanine Notes Payable and Outside Members’ Notes Payable On June 7, 2017, the Company’s consolidated entity in which it has a 60% ownership interest and that owns 767 Fifth Avenue (the General Motors Building) located in New York City completed the refinancing of the indebtedness that had been secured by direct and indirect interests in the property. The new mortgage financing has a principal amount of $2.3 billion, bears interest at a fixed interest rate of 3.43% per annum and matures on June 9, 2027. The loan requires monthly interest-only payments during the 10-year term of the loan, with the entire principal amount being due at maturity. The refinanced indebtedness consisted of (1) mortgage loans payable collateralized by the property aggregating $1.3 billion, (2) mezzanine loans payable aggregating $306.0 million, (3) additional mezzanine loans payable aggregating $294.0 million and (4) member loans aggregating $450.0 million with outstanding accrued interest payable totaling approximately $425.0 million. The mortgage loans required monthly interest-only payments at a weighted-average fixed interest rate of 5.95% per annum and were scheduled to mature on October 7, 2017. The mezzanine loans required interest-only payments at a weighted-average fixed interest rate of 6.02% per annum and were scheduled to mature on October 7, 2017. In addition, a subsidiary of the consolidated entity had acquired a lender’s interest in certain other mezzanine loans assumed during the acquisition of the property having an aggregate principal amount of $294.0 million and a stated interest rate of 6.02% per annum for a purchase price of approximately $263.1 million in cash. These mezzanine loans payable had been eliminated in consolidation and were canceled upon the refinancing of the indebtedness. The member loans bore interest at a fixed rate of 11.0% per annum and were scheduled to mature on June 9, 2017. A portion of the original purchase price of the property was financed with loans from the members on a pro rata basis equal to their percentage interest in the consolidated entity. The Company had eliminated in consolidation its member loan totaling $270.0 million and its share of the related accrued interest payable of approximately $255.0 million at the date of the refinancing. The remaining outside members’ notes payable and related accrued interest payable totaling $180.0 million and approximately $170.0 million, respectively, at the date of the refinancing had been reflected as Outside Members’ Notes Payable and within Accrued Interest Payable, respectively, on the Company’s Consolidated Balance Sheets. The net proceeds from the new financing were used to repay all of the outstanding accrued interest payable on the member loans and a portion of the outstanding principal balance of the member loans totaling approximately $176.1 million. In connection with the refinancing, the members of the Company’s consolidated entity contributed the remaining balance of the member notes payable totaling approximately $273.9 million (of which the Company’s share of approximately $164.4 million had been eliminated in consolidation) to equity in the consolidated entity (See Note 8). There was no prepayment penalty associated with the repayments. The Company recognized a gain from early extinguishment of debt totaling approximately $14.6 million primarily consisting of the acceleration of the remaining balance related to historical fair value debt adjustments. Credit Facility On April 24, 2017, Boston Properties Limited Partnership amended and restated its revolving credit agreement (as amended and restated, the “2017 Credit Facility”). Among other things, the 2017 Credit Facility (1) increased the total commitment of the revolving line of credit (the “Revolving Facility”) from $1.0 billion to $1.5 billion, (2) extended the maturity date from July 26, 2018 to April 24, 2022, (3) reduced the per annum variable interest rates, and (4) added a $500.0 million delayed draw term loan facility (the “Delayed Draw Facility”) that permits Boston Properties Limited Partnership, until the first anniversary of the closing date, to draw upon up to four times a minimum of $50.0 million (or, if less, the unused delayed draw term commitments), provided that amounts drawn under the Delayed Draw Facility and subsequently repaid may not be borrowed again. In addition, Boston Properties Limited Partnership may increase the total commitment under the 2017 Credit Facility by up to $500.0 million through increases in the Revolving Facility or the Delayed Draw Facility, or both, subject to syndication of the increase and other conditions. At Boston Properties Limited Partnership’s option, loans under the Revolving Facility and Delayed Draw Facility will bear interest at a rate per annum equal to (1) (a) in the case of loans denominated in Dollars, Euro or Sterling, LIBOR, and (b) in the case of loans denominated in Canadian Dollars, CDOR, in each case, plus a margin ranging from 77.5 to 155 basis points for the Revolving Commitment and 85 to 175 basis points for the Delayed Draw Facility, based on Boston Properties Limited Partnership’s credit rating or (2) an alternate base rate equal to the greatest of (x) the Administrative Agent’s prime rate, (y) the Federal Funds rate plus 0.50% or (z) LIBOR for a one-month period plus 1.00%, in each case, plus a margin ranging from 0 to 55 basis points for the Revolving Facility and 0 to 75 basis points for the Delayed Draw Facility, based on Boston Properties Limited Partnership’s credit rating. The 2017 Credit Facility also contains a competitive bid option for up to 65% of the Revolving Facility that allows banks that are part of the lender consortium to bid to make loan advances to Boston Properties Limited Partnership at a reduced interest rate. In addition, Boston Properties Limited Partnership is obligated to pay (1) in quarterly installments a facility fee on the total commitment under the Revolving Facility at a rate per annum ranging from 0.10% to 0.30% based on Boston Properties Limited Partnership’s credit rating, (2) an annual fee on the undrawn amount of each letter of credit equal to the LIBOR margin on the Revolving Facility and (3) a fee on the unused commitments under the Delayed Draw Facility equal to 0.15% per annum. Based on Boston Properties Limited Partnership’s current credit rating, (1) the applicable Eurocurrency margins for the Revolving Facility and Delayed Draw Facility are 87.5 basis points and 95 basis points, respectively, (2) the alternate base rate margin is 0 basis points for each of the Revolving Facility and Delayed Draw Facility and (3) the facility fee on the Revolving Facility commitment is 0.15% per annum. The 2017 Credit Facility contains customary representations and warranties, affirmative and negative covenants and events of default provisions, including failure to pay indebtedness, breaches of covenants, and bankruptcy and other insolvency events, which could result in the acceleration of all amounts and cancellation of all commitments outstanding under the Credit Agreement. Among other covenants, the 2017 Credit Facility requires that Boston Properties Limited Partnership maintain on an ongoing basis: (1) a leverage ratio not to exceed 60%, however, the leverage ratio may increase to no greater than 65% provided that it is reduced back to 60% within one year, (2) a secured debt leverage ratio not to exceed 55%, (3) a fixed charge coverage ratio of at least 1.40, (4) an unsecured debt leverage ratio not to exceed 60%, however, the unsecured debt leverage ratio may increase to no greater than 65% provided that it is reduced to 60% within one year, (5) an unsecured debt interest coverage ratio of at least 1.75 and (6) limitations on permitted investments.

5. Debt

Mortgage Notes Payable, Net, Mezzanine Notes Payable and Outside Members’ Notes Payable

On June 7, 2017, the Company’s consolidated entity in which it has a 60% ownership interest and that owns 767 Fifth Avenue (the General Motors Building) located in New York City completed the refinancing of the indebtedness that had been secured by direct and indirect interests in the property. The new mortgage financing has a principal amount of $2.3 billion, bears interest at a fixed interest rate of 3.43% per annum and matures on June 9, 2027. The loan requires monthly interest-only payments during the 10-year term of the loan, with the entire principal amount being due at maturity.

The refinanced indebtedness consisted of (1) mortgage loans payable collateralized by the property aggregating $1.3 billion, (2) mezzanine loans payable aggregating $306.0 million, (3) additional mezzanine loans payable aggregating $294.0 million and (4) member loans aggregating $450.0 million with outstanding accrued interest payable totaling approximately $425.0 million. The mortgage loans required monthly interest-only payments at a weighted-average fixed interest rate of 5.95% per annum and were scheduled to mature on October 7, 2017. The mezzanine loans required interest-only payments at a weighted-average fixed interest rate of 6.02% per annum and were scheduled to mature on October 7, 2017. In addition, a subsidiary of the consolidated entity had acquired a lender’s interest in certain other mezzanine loans assumed during the acquisition of the property having an aggregate principal amount of $294.0 million and a stated interest rate of 6.02% per annum for a purchase price of approximately $263.1 million in cash. These mezzanine loans payable had been eliminated in consolidation and were canceled upon the refinancing of the indebtedness. The member loans bore interest at a fixed rate of 11.0% per annum and were scheduled to mature on June 9, 2017. A portion of the original purchase price of the property was financed with loans from the members on a pro rata basis equal to their percentage interest in the consolidated entity. The Company had eliminated in consolidation its member loan totaling $270.0 million and its share of the related accrued interest payable of approximately $255.0 million at the date of the refinancing. The remaining outside members’ notes payable and related accrued interest payable totaling $180.0 million and approximately $170.0 million, respectively, at the date of the refinancing had been reflected as Outside Members’ Notes Payable and within Accrued Interest Payable, respectively, on the Company’s Consolidated Balance Sheets. The net proceeds from the new financing were used to repay all of the outstanding accrued interest payable on the member loans and a portion of the outstanding principal balance of the member loans totaling approximately $176.1 million. In connection with the refinancing, the members of the Company’s consolidated entity contributed the remaining balance of the member notes payable totaling approximately $273.9 million (of which the Company’s share of approximately $164.4 million had been eliminated in consolidation) to equity in the consolidated entity (See Note 8). There was no prepayment penalty associated with the repayments. The Company recognized a gain from early extinguishment of debt totaling approximately $14.6 million primarily consisting of the acceleration of the remaining balance related to historical fair value debt adjustments.

Credit Facility

On April 24, 2017, Boston Properties Limited Partnership amended and restated its revolving credit agreement (as amended and restated, the “2017 Credit Facility”). Among other things, the 2017 Credit Facility (1) increased the total commitment of the revolving line of credit (the “Revolving Facility”) from $1.0 billion to $1.5 billion, (2) extended the maturity date from July 26, 2018 to April 24, 2022, (3) reduced the per annum variable interest rates, and (4) added a $500.0 million delayed draw term loan facility (the “Delayed Draw Facility”) that permits Boston Properties Limited Partnership, until the first anniversary of the closing date, to draw upon up to four times a minimum of $50.0 million (or, if less, the unused delayed draw term commitments), provided that amounts drawn under the Delayed Draw Facility and subsequently repaid may not be borrowed again. In addition, Boston Properties Limited Partnership may increase the total commitment under the 2017 Credit Facility by up to $500.0 million through increases in the Revolving Facility or the Delayed Draw Facility, or both, subject to syndication of the increase and other conditions.

At Boston Properties Limited Partnership’s option, loans under the Revolving Facility and Delayed Draw Facility will bear interest at a rate per annum equal to (1) (a) in the case of loans denominated in Dollars, Euro or Sterling, LIBOR, and (b) in the case of loans denominated in Canadian Dollars, CDOR, in each case, plus a margin ranging from 77.5 to 155 basis points for the Revolving Commitment and 85 to 175 basis points for the Delayed Draw Facility, based on Boston Properties Limited Partnership’s credit rating or (2) an alternate base rate equal to the greatest of (x) the Administrative Agent’s prime rate, (y) the Federal Funds rate plus 0.50% or (z) LIBOR for a one-month period plus 1.00%, in each case, plus a margin ranging from 0 to 55 basis points for the Revolving Facility and 0 to 75 basis points for the Delayed Draw Facility, based on Boston Properties Limited Partnership’s credit rating. The 2017 Credit Facility also contains a competitive bid option for up to 65% of the Revolving Facility that allows banks that are part of the lender consortium to bid to make loan advances to Boston Properties Limited Partnership at a reduced interest rate.

In addition, Boston Properties Limited Partnership is obligated to pay (1) in quarterly installments a facility fee on the total commitment under the Revolving Facility at a rate per annum ranging from 0.10% to 0.30% based on Boston Properties Limited Partnership’s credit rating, (2) an annual fee on the undrawn amount of each letter of credit equal to the LIBOR margin on the Revolving Facility and (3) a fee on the unused commitments under the Delayed Draw Facility equal to 0.15% per annum.

Based on Boston Properties Limited Partnership’s current credit rating, (1) the applicable Eurocurrency margins for the Revolving Facility and Delayed Draw Facility are 87.5 basis points and 95 basis points, respectively, (2) the alternate base rate margin is 0 basis points for each of the Revolving Facility and Delayed Draw Facility and (3) the facility fee on the Revolving Facility commitment is 0.15% per annum.

The 2017 Credit Facility contains customary representations and warranties, affirmative and negative covenants and events of default provisions, including failure to pay indebtedness, breaches of covenants, and bankruptcy and other insolvency events, which could result in the acceleration of all amounts and cancellation of all commitments outstanding under the Credit Agreement. Among other covenants, the 2017 Credit Facility requires that Boston Properties Limited Partnership maintain on an ongoing basis: (1) a leverage ratio not to exceed 60%, however, the leverage ratio may increase to no greater than 65% provided that it is reduced back to 60% within one year, (2) a secured debt leverage ratio not to exceed 55%, (3) a fixed charge coverage ratio of at least 1.40, (4) an unsecured debt leverage ratio not to exceed 60%, however, the unsecured debt leverage ratio may increase to no greater than 65% provided that it is reduced to 60% within one year, (5) an unsecured debt interest coverage ratio of at least 1.75 and (6) limitations on permitted investments.