Subsequent Events	3 Months Ended
Mar. 31, 2013
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On April 1, 2013, the Company was designated as the Owner's Representative by Harvard Planning and Project Management to provide development management services for the new Health and Life Sciences Facility. On April 1, 2013, the Company used available cash to repay the mortgage loan collateralized by its 140 Kendrick Street property located in Needham, Massachusetts totaling approximately $47.6 million. The mortgage loan bore interest at a fixed rate of 7.51% per annum and was scheduled to mature on July 1, 2013. There was no prepayment penalty. On April 2, 2013, the Company issued 24,231 LTIP units, 38,926 2013 MYLTIP Units and 50,847 non-qualified stock options under the 2012 Plan to Owen D. Thomas, the Company's new Chief Executive Officer, pursuant to his employment agreement. On April 4, 2013, a joint venture in which the Company has a 50% interest obtained construction financing collateralized by its Annapolis Junction Building Seven development project located in Annapolis, Maryland totaling $22.0 million. The construction financing bears interest at a variable rate equal to LIBOR plus 1.65% per annum and matures on April 4, 2016, with two, one-year extension options, subject to certain conditions. On April 10, 2013, the Company acquired the Mountain View Research Park and Mountain View Technology Park properties from its Value-Added Fund for an aggregate purchase price of approximately $233.5 million. In conjunction with the acquisition, the Value-Added Fund repaid the mortgage loans collateralized by the Mountain View Research Park and Mountain View Technology Park properties totaling approximately $90.0 million and $20.0 million, respectively, as well as the outstanding loans payable to the Company's Operating Partnership totaling approximately $8.6 million and $3.7 million, respectively. On April 10, 2013, a joint venture in which the Company has a 60% interest executed a binding contract for the sale of its 125 West 55th Street property located in New York City for a sale price of $470.0 million, subject to the assumption by the buyer of the mortgage loan collateralized by the property totaling approximately $199.0 million. 125 West 55th Street is a Class A office property totaling approximately 588,000 net rentable square feet. The sale price for the property exceeds its carrying value. 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 April 11, 2013, the Company's Operating Partnership completed a public offering of $500.0 million in aggregate principal amount of its 3.125% senior unsecured notes due 2023. The notes were priced at 99.379% of the principal amount to yield an effective rate (including financing fees) of 3.279% to maturity. The notes will mature on September 1, 2023, unless earlier redeemed. The aggregate net proceeds from the offering were approximately $492.5 million after deducting underwriting discounts and transaction expenses. On April 15, 2013, the Company announced that holders of its Operating Partnership's 3.75% Exchangeable Senior Notes due 2036 (the “Notes”) have the right to surrender their Notes for purchase by the Operating Partnership (the “Put Right”) on May 18, 2013. The opportunity to exercise the Put Right will expire at 5:00 p.m., New York City time, on May 13, 2013. On April 15, 2013, the Company also announced that the Operating Partnership issued a notice of redemption to the holders of the Notes to redeem, on May 18, 2013 (the “Redemption Date”), all of the Notes outstanding on the Redemption Date. In connection with the redemption, holders of the Notes have the right to exchange their Notes prior to 5:00 p.m., New York City time, on May 16, 2013. Notes with respect to which the Put Right is not exercised (or is exercised and subsequently withdrawn) and that are not surrendered for exchange prior to 5:00 p.m., New York City time, on May 16, 2013, will be redeemed by the Operating Partnership at a redemption price equal to 100% of the principal amount of the Notes plus accrued and unpaid interest thereon to, but excluding, the Redemption Date. As of April 30, 2012, there was approximately $450.0 million aggregate principal amount of the Notes outstanding. On April 25, 2013, the Company commenced construction of its 601 Massachusetts Avenue development project totaling approximately 478,000 net rentable square feet located in Washington, DC.

Subsequent Events

On April 1, 2013, the Company was designated as the Owner's Representative by Harvard Planning and Project Management to provide development management services for the new Health and Life Sciences Facility.

On April 1, 2013, the Company used available cash to repay the mortgage loan collateralized by its 140 Kendrick Street property located in Needham, Massachusetts totaling approximately $47.6 million. The mortgage loan bore interest at a fixed rate of 7.51% per annum and was scheduled to mature on July 1, 2013. There was no prepayment penalty.

On April 2, 2013, the Company issued 24,231 LTIP units, 38,926 2013 MYLTIP Units and 50,847 non-qualified stock options under the 2012 Plan to Owen D. Thomas, the Company's new Chief Executive Officer, pursuant to his employment agreement.

On April 4, 2013, a joint venture in which the Company has a 50% interest obtained construction financing collateralized by its Annapolis Junction Building Seven development project located in Annapolis, Maryland totaling $22.0 million. The construction financing bears interest at a variable rate equal to LIBOR plus 1.65% per annum and matures on April 4, 2016, with two, one-year extension options, subject to certain conditions.

On April 10, 2013, the Company acquired the Mountain View Research Park and Mountain View Technology Park properties from its Value-Added Fund for an aggregate purchase price of approximately $233.5 million. In conjunction with the acquisition, the Value-Added Fund repaid the mortgage loans collateralized by the Mountain View Research Park and Mountain View Technology Park properties totaling approximately $90.0 million and $20.0 million, respectively, as well as the outstanding loans payable to the Company's Operating Partnership totaling approximately $8.6 million and $3.7 million, respectively.

On April 10, 2013, a joint venture in which the Company has a 60% interest executed a binding contract for the sale of its 125 West 55th Street property located in New York City for a sale price of $470.0 million, subject to the assumption by the buyer of the mortgage loan collateralized by the property totaling approximately $199.0 million. 125 West 55th Street is a Class A office property totaling approximately 588,000 net rentable square feet. The sale price for the property exceeds its carrying value. 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 April 11, 2013, the Company's Operating Partnership completed a public offering of $500.0 million in aggregate principal amount of its 3.125% senior unsecured notes due 2023. The notes were priced at 99.379% of the principal amount to yield an effective rate (including financing fees) of 3.279% to maturity. The notes will mature on September 1, 2023, unless earlier redeemed. The aggregate net proceeds from the offering were approximately $492.5 million after deducting underwriting discounts and transaction expenses.

On April 15, 2013, the Company announced that holders of its Operating Partnership's 3.75% Exchangeable Senior Notes due 2036 (the “Notes”) have the right to surrender their Notes for purchase by the Operating Partnership (the “Put Right”) on May 18, 2013. The opportunity to exercise the Put Right will expire at 5:00 p.m., New York City time, on May 13, 2013. On April 15, 2013, the Company also announced that the Operating Partnership issued a notice of redemption to the holders of the Notes to redeem, on May 18, 2013 (the “Redemption Date”), all of the Notes outstanding on the Redemption Date. In connection with the redemption, holders of the Notes have the right to exchange their Notes prior to 5:00 p.m., New York City time, on May 16, 2013. Notes with respect to which the Put Right is not exercised (or is exercised and subsequently withdrawn) and that are not surrendered for exchange prior to 5:00 p.m., New York City time, on May 16, 2013, will be redeemed by the Operating Partnership at a redemption price equal to 100% of the principal amount of the Notes plus accrued and unpaid interest thereon to, but excluding, the Redemption Date. As of April 30, 2012, there was approximately $450.0 million aggregate principal amount of the Notes outstanding.

On April 25, 2013, the Company commenced construction of its 601 Massachusetts Avenue development project totaling approximately 478,000 net rentable square feet located in Washington, DC.