Subsequent Events	9 Months Ended
Sep. 30, 2014
Subsequent Events [Abstract]
Subsequent Events	13. Subsequent Events On October 2, 2014, the Company completed the sale of its Patriots Park properties located in Reston, Virginia for a gross sale price of $321.0 million, which exceeded the carrying value of the properties. Patriots Park consists of three Class A office properties aggregating approximately 706,000 net rentable square feet. On October 16, 2014, the Company's Operating Partnership released to the holders 27,773 Series Four Preferred Units that were previously subject to a security interest under a pledge agreement (See Note 8). On November 5, 2014, the Company's Operating Partnership redeemed such units for cash totaling approximately $1.4 million. An aggregate of 12,667 Series Four Preferred Units remain outstanding and subject to the security interest under the pledge agreement. On October 22, 2014, MIT exercised its right to purchase the Company’s 415 Main Street property (formerly Seven Cambridge Center) located in Cambridge, Massachusetts on February 1, 2016 for approximately $106 million. As part of its lease signed on July 14, 2004, MIT was granted a fixed price option to purchase the building at the beginning of the 11th lease year. 415 Main Street is an Office/Technical property with approximately 231,000 net rentable square feet occupied by the Broad Institute. The sale is subject to the satisfaction of customary closing conditions and there can be no assurance that the sale will be consummated on the terms currently contemplated or at all. On October 24, 2014, the Company completed the sale of a parcel of land at 130 Third Avenue in Waltham, Massachusetts that is permitted for 129,000 square feet for a sale price of approximately $14.3 million, which exceeded the carrying value of the property. On October 24, 2014, a joint venture in which the Company has a 50% interest extended the loan collateralized by its Annapolis Junction Building Six property. At the time of the extension, the outstanding balance of the construction loan totaled approximately $13.9 million and bore interest at a variable rate equal to LIBOR plus 1.65% per annum and was scheduled to mature on November 17, 2014. The extended loan has a total commitment amount of $16.4 million, bears interest at a variable rate equal to LIBOR plus 2.25% per annum and matures on November 17, 2015. Annapolis Junction Building Six is a Class A office property with approximately 119,000 net rentable square feet located in Annapolis, Maryland. On October 30, 2014, the Company completed the sale of a 45% interest in each of 601 Lexington Avenue in New York City and Atlantic Wharf Office Building and 100 Federal Street in Boston for an aggregate gross sales price of approximately $1.827 billion in cash, less the partner’s pro rata share of the indebtedness secured by 601 Lexington Avenue, subject to certain prorations and adjustments. In connection with the sale, the Company formed a joint venture for each property with the buyer and will provide customary property management and leasing services to the joint ventures. 601 Lexington Avenue is a 1,669,000 square foot Class A office complex located in Midtown Manhattan. The property consists of a 59-story tower as well as a six-story low-rise office and retail building. The property is subject to existing mortgage indebtedness of approximately $712.9 million. The Atlantic Wharf Office Building is a 791,000 square foot Class A office tower located on Boston's Waterfront. 100 Federal Street is a 1,323,000 square foot Class A office tower located in Boston's Financial District. The transaction did not qualify as a sale of real estate for financial reporting purposes as the Company continues to control the joint ventures and will therefore continue to account for the properties on a consolidated basis in its financial statements. On November 1, 2014, the Company partially placed in-service 535 Mission Street, a Class A office project with approximately 307,000 net rentable square feet located in San Francisco, California. On November 3, 2014, the Company's Operating Partnership issued notices to redeem on December 15, 2014 (i) $300.0 million in aggregate principal amount of its 5.625% senior notes due 2015, which were scheduled to mature on April 15, 2015 (the “5.625% Notes”), and (ii) $250.0 million in aggregate principal amount of its 5.000% senior notes due 2015, which were scheduled to mature on June 1, 2015 (the “5.000% Notes”). The redemption price for each series of notes will equal the sum of (a) the present values as of the redemption date of the remaining scheduled payments of principal and interest from the redemption date to maturity (excluding any accrued and unpaid interest) discounted on a semi-annual basis at a rate equal to the yield to maturity of a comparable United States Treasury security plus (i) in the case of the 5.625% Notes, 0.35%, or (ii) in the case of the 5.000% Notes, 0.25%, plus (b) accrued interest to, but excluding, December 15, 2014, as provided in the applicable indenture. The redemption price for each series of notes will be calculated three business days prior to the redemption date and will be payable on December 15, 2014 in accordance with the terms of the applicable indentures. On November 6, 2014, the Company entered into an option agreement pursuant to which the Company has been granted an option to purchase real property located at 425 Fourth Street in San Francisco, California. In connection with the execution of the agreement, the Company paid a non-refundable option payment to the current owner of $1.0 million. The Company intends to pursue the entitlements necessary to develop the property. The purchase price has not been determined and is dependent on the entitlements obtained. There can be no assurance that the Company will be successful in obtaining the desired entitlements or that it will ultimately determine to exercise the option.

13. Subsequent Events

On October 2, 2014, the Company completed the sale of its Patriots Park properties located in Reston, Virginia for a gross sale price of $321.0 million, which exceeded the carrying value of the properties. Patriots Park consists of three Class A office properties aggregating approximately 706,000 net rentable square feet.

On October 16, 2014, the Company's Operating Partnership released to the holders 27,773 Series Four Preferred Units that were previously subject to a security interest under a pledge agreement (See Note 8). On November 5, 2014, the Company's Operating Partnership redeemed such units for cash totaling approximately $1.4 million. An aggregate of 12,667 Series Four Preferred Units remain outstanding and subject to the security interest under the pledge agreement.

On October 22, 2014, MIT exercised its right to purchase the Company’s 415 Main Street property (formerly Seven Cambridge Center) located in Cambridge, Massachusetts on February 1, 2016 for approximately $106 million. As part of its lease signed on July 14, 2004, MIT was granted a fixed price option to purchase the building at the beginning of the 11th lease year. 415 Main Street is an Office/Technical property with approximately 231,000 net rentable square feet occupied by the Broad Institute. The sale is subject to the satisfaction of customary closing conditions and there can be no assurance that the sale will be consummated on the terms currently contemplated or at all.

On October 24, 2014, the Company completed the sale of a parcel of land at 130 Third Avenue in Waltham, Massachusetts that is permitted for 129,000 square feet for a sale price of approximately $14.3 million, which exceeded the carrying value of the property.

On October 24, 2014, a joint venture in which the Company has a 50% interest extended the loan collateralized by its Annapolis Junction Building Six property. At the time of the extension, the outstanding balance of the construction loan totaled approximately $13.9 million and bore interest at a variable rate equal to LIBOR plus 1.65% per annum and was scheduled to mature on November 17, 2014. The extended loan has a total commitment amount of $16.4 million, bears interest at a variable rate equal to LIBOR plus 2.25% per annum and matures on November 17, 2015. Annapolis Junction Building Six is a Class A office property with approximately 119,000 net rentable square feet located in Annapolis, Maryland.

On October 30, 2014, the Company completed the sale of a 45% interest in each of 601 Lexington Avenue in New York City and Atlantic Wharf Office Building and 100 Federal Street in Boston for an aggregate gross sales price of approximately $1.827 billion in cash, less the partner’s pro rata share of the indebtedness secured by 601 Lexington Avenue, subject to certain prorations and adjustments. In connection with the sale, the Company formed a joint venture for each property with the buyer and will provide customary property management and leasing services to the joint ventures. 601 Lexington Avenue is a 1,669,000 square foot Class A office complex located in Midtown Manhattan. The property consists of a 59-story tower as well as a six-story low-rise office and retail building. The property is subject to existing mortgage indebtedness of approximately $712.9 million. The Atlantic Wharf Office Building is a 791,000 square foot Class A office tower located on Boston's Waterfront. 100 Federal Street is a 1,323,000 square foot Class A office tower located in Boston's Financial District. The transaction did not qualify as a sale of real estate for financial reporting purposes as the Company continues to control the joint ventures and will therefore continue to account for the properties on a consolidated basis in its financial statements.

On November 1, 2014, the Company partially placed in-service 535 Mission Street, a Class A office project with approximately 307,000 net rentable square feet located in San Francisco, California.

On November 3, 2014, the Company's Operating Partnership issued notices to redeem on December 15, 2014 (i) $300.0 million in aggregate principal amount of its 5.625% senior notes due 2015, which were scheduled to mature on April 15, 2015 (the “5.625% Notes”), and (ii) $250.0 million in aggregate principal amount of its 5.000% senior notes due 2015, which were scheduled to mature on June 1, 2015 (the “5.000% Notes”). The redemption price for each series of notes will equal the sum of (a) the present values as of the redemption date of the remaining scheduled payments of principal and interest from the redemption date to maturity (excluding any accrued and unpaid interest) discounted on a semi-annual basis at a rate equal to the yield to maturity of a comparable United States Treasury security plus (i) in the case of the 5.625% Notes, 0.35%, or (ii) in the case of the 5.000% Notes, 0.25%, plus (b) accrued interest to, but excluding, December 15, 2014, as provided in the applicable indenture. The redemption price for each series of notes will be calculated three business days prior to the redemption date and will be payable on December 15, 2014 in accordance with the terms of the applicable indentures.

On November 6, 2014, the Company entered into an option agreement pursuant to which the Company has been granted an option to purchase real property located at 425 Fourth Street in San Francisco, California. In connection with the execution of the agreement, the Company paid a non-refundable option payment to the current owner of $1.0 million. The Company intends to pursue the entitlements necessary to develop the property. The purchase price has not been determined and is dependent on the entitlements obtained. There can be no assurance that the Company will be successful in obtaining the desired entitlements or that it will ultimately determine to exercise the option.