Subsequent Events	6 Months Ended
Jun. 30, 2015
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events From July 1, 2015 through August 7, 2015, 767 Fifth Partners LLC, which is the consolidated entity (in which the Company has a 60% interest and owns 767 Fifth Avenue (the General Motors Building) in New York City), entered into four forward-starting interest rate swap contracts which fix the 10-year swap rate at a weighted-average rate of 2.813% per annum on notional amounts aggregating $100.0 million. The interest rate swap contracts were entered into in advance of a financing with a target commencement date in June 2017 and maturity in June 2027 (See Note 5). In addition, the Company entered into one forward-starting interest rate swap contract which fixes the 10-year swap rate at a rate of 2.532% per annum on a notional amount of $25.0 million. The interest rate swap contract was entered into in advance of a financing with a target commencement date in September 2016 and maturity in September 2026 (See Note 5). On July 29, 2015, a consolidated entity in which the Company has a 50% interest executed a binding agreement for the sale of 505 9th Street, N.W. located in Washington, DC, for approximately $318.0 million (which exceeds its carrying value), including the assumption by the buyer of approximately $117.0 million of mortgage indebtedness. 505 9th Street, N.W. is an approximately 322,000 net rentable square foot Class A office building. 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 July 31, 2015, the Company entered into a 99-year ground and air rights lease (the “Lease”) with the Massachusetts Department of Transportation (“MDOT”) with respect to the parking garage located at 100 Clarendon Street (the “Clarendon Garage”) and the concourse level of the Massachusetts Bay Transportation Authority’s Back Bay Station (the “Station”). The Lease amends and restates the air rights lease which the Company had assumed in 2010 at the time it acquired its interests in both the Clarendon Garage and the office tower located at 200 Clarendon Street (formerly the John Hancock Tower). The previous lease had 45 years remaining in its term. Upon execution of the Lease, the Company made a $5.0 million option payment which provides the Company with options under the Lease to acquire certain air rights above both the Clarendon Garage and the Station; the amount of developable square footage associated with the air rights will be determined at a later date. In consideration for MDOT entering into the new Lease, the Company has agreed to make improvements to the Station in an amount of $32.0 million, which amount will be credited against the Company’s rental obligations under the Lease. On August 6, 2015, the Company executed an amendment to the joint venture agreement for its Fountain Square property located in Reston, Virginia. The amendment requires the Company to acquire its partner's nominal 50% interest on September 15, 2015 for approximately $100.9 million in cash (See Note 7).

12. Subsequent Events

From July 1, 2015 through August 7, 2015, 767 Fifth Partners LLC, which is the consolidated entity (in which the Company has a 60% interest and owns 767 Fifth Avenue (the General Motors Building) in New York City), entered into four forward-starting interest rate swap contracts which fix the 10-year swap rate at a weighted-average rate of 2.813% per annum on notional amounts aggregating $100.0 million. The interest rate swap contracts were entered into in advance of a financing with a target commencement date in June 2017 and maturity in June 2027 (See Note 5). In addition, the Company entered into one forward-starting interest rate swap contract which fixes the 10-year swap rate at a rate of 2.532% per annum on a notional amount of $25.0 million. The interest rate swap contract was entered into in advance of a financing with a target commencement date in September 2016 and maturity in September 2026 (See Note 5).

On July 29, 2015, a consolidated entity in which the Company has a 50% interest executed a binding agreement for the sale of 505 9th Street, N.W. located in Washington, DC, for approximately $318.0 million (which exceeds its carrying value), including the assumption by the buyer of approximately $117.0 million of mortgage indebtedness. 505 9th Street, N.W. is an approximately 322,000 net rentable square foot Class A office building. 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 July 31, 2015, the Company entered into a 99-year ground and air rights lease (the “Lease”) with the Massachusetts Department of Transportation (“MDOT”) with respect to the parking garage located at 100 Clarendon Street (the “Clarendon Garage”) and the concourse level of the Massachusetts Bay Transportation Authority’s Back Bay Station (the “Station”). The Lease amends and restates the air rights lease which the Company had assumed in 2010 at the time it acquired its interests in both the Clarendon Garage and the office tower located at 200 Clarendon Street (formerly the John Hancock Tower). The previous lease had 45 years remaining in its term. Upon execution of the Lease, the Company made a $5.0 million option payment which provides the Company with options under the Lease to acquire certain air rights above both the Clarendon Garage and the Station; the amount of developable square footage associated with the air rights will be determined at a later date. In consideration for MDOT entering into the new Lease, the Company has agreed to make improvements to the Station in an amount of $32.0 million, which amount will be credited against the Company’s rental obligations under the Lease.

On August 6, 2015, the Company executed an amendment to the joint venture agreement for its Fountain Square property located in Reston, Virginia. The amendment requires the Company to acquire its partner's nominal 50% interest on September 15, 2015 for approximately $100.9 million in cash (See Note 7).