Subsequent Events	12 Months Ended
Dec. 31, 2012
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On January 28, 2013, the Company's Compensation Committee approved multi-year long-term incentive program (MYLTIP) awards under the Company's 2012 Plan to officers and employees of the Company. MYLTIP awards utilize TRS over a three-year measurement period, on an annualized, compounded basis, as the performance metric. Earned awards will be based on the Company's TRS relative to (i) the Cohen & Steers Realty Majors Portfolio Index (50% weight) and (ii) the NAREIT Office Index adjusted to exclude the Company (50% weight). Earned awards will range from zero to a maximum of approximately $30.7 million depending on the Company's TRS relative to the two indices, with four tiers (threshold: approximately $5.1 million; target: approximately $10.2 million; high: approximately $20.5 million; exceptional: approximately $30.7 million) and linear interpolation between tiers. Earned awards measured on the basis of relative TRS performance are subject to an absolute TRS component in the form of relatively simple modifiers that (A) reduce the level of earned awards in the event the Company's annualized TRS is less than 2% and (B) cause some awards to be earned in the event the Company's annualized TRS is more than 10% even though on a relative basis alone the Company's TRS would not result in any earned awards. Earned awards (if any) will vest 25% on February 4, 2016, 25% on February 4, 2017 and 50% on February 4, 2018, based on continued employment. Vesting will be accelerated in the event of a change in control, termination of employment by the Company without cause, termination of employment by the award recipient for good reason, death, disability or retirement. If there is a change of control prior to February 4, 2016, earned awards will be calculated based on TRS performance up to the date of the change of control. MYLTIP awards are in the form of LTIP Units issued on the grant date which (i) are subject to forfeiture to the extent awards are not earned and (ii) prior to the performance measurement date are only entitled to one-tenth (10%) of the regular quarterly distributions payable on common partnership units. Under the Financial Accounting Standards Board's Accounting Standards Codification (“ASC”) 718 “Compensation-Stock Compensation” the MYLTIP awards have an aggregate value of approximately $8.1 million, which amount will generally be amortized into earnings over the five-year plan period under the graded vesting method. On January 29, 2013, the Company entered into an agreement to acquire a parcel of land located in Reston, Virginia for a purchase price of approximately $27.0 million in cash. There can be no assurance that the acquisition will be completed on the terms currently contemplated or at all. On February 1, 2013, the Company issued 35,087 shares of restricted common stock, 153,006 LTIP units and 201,373 non-qualified stock options under the 2012 Plan to certain employees of the Company. On February 5, 2013, the Company used available cash to repay the mortgage loan collateralized by its Kingstowne One property located in Alexandria, Virginia totaling approximately $17.0 million. The mortgage loan bore interest at a fixed rate of 5.96% per annum and was scheduled to mature on May 5, 2013. There was no prepayment penalty. On February 6, 2013, the Company completed the acquisition of 535 Mission Street, a development site, in San Francisco, California for an aggregate purchase price of approximately $71.0 million in cash, including work completed and materials purchased to date. When completed, 535 Mission Street will consist of a 27-story, Class A office tower with approximately 307,000 net rentable square feet of office and retail space. The Company has commenced development of the project. On February 7, 2013, the partner in the Company's joint venture that is pursuing the acquisition of land in San Francisco, California which could support the office tower known as Transbay Tower, issued a notice that it has elected under the joint venture agreement to reduce its nominal ownership interest in the venture from 50% to 5%. On February 26, 2013, the Company issued a notice to such partner electing to proceed with the venture on that basis. As a result, the Company has a 95% nominal interest in, and expects to consolidate, the joint venture.

Subsequent Events

On January 28, 2013, the Company's Compensation Committee approved multi-year long-term incentive program (MYLTIP) awards under the Company's 2012 Plan to officers and employees of the Company. MYLTIP awards utilize TRS over a three-year measurement period, on an annualized, compounded basis, as the performance metric. Earned awards will be based on the Company's TRS relative to (i) the Cohen & Steers Realty Majors Portfolio Index (50% weight) and (ii) the NAREIT Office Index adjusted to exclude the Company (50% weight). Earned awards will range from zero to a maximum of approximately $30.7 million depending on the Company's TRS relative to the two indices, with four tiers (threshold: approximately $5.1 million; target: approximately $10.2 million; high: approximately $20.5 million; exceptional: approximately $30.7 million) and linear interpolation between tiers. Earned awards measured on the basis of relative TRS performance are subject to an absolute TRS component in the form of relatively simple modifiers that (A) reduce the level of earned awards in the event the Company's annualized TRS is less than 2% and (B) cause some awards to be earned in the event the Company's annualized TRS is more than 10% even though on a relative basis alone the Company's TRS would not result in any earned awards.

Earned awards (if any) will vest 25% on February 4, 2016, 25% on February 4, 2017 and 50% on February 4, 2018, based on continued employment. Vesting will be accelerated in the event of a change in control, termination of employment by the Company without cause, termination of employment by the award recipient for good reason, death, disability or retirement. If there is a change of control prior to February 4, 2016, earned awards will be calculated based on TRS performance up to the date of the change of control. MYLTIP awards are in the form of LTIP Units issued on the grant date which (i) are subject to forfeiture to the extent awards are not earned and (ii) prior to the performance measurement date are only entitled to one-tenth (10%) of the regular quarterly distributions payable on common partnership units.

Under the Financial Accounting Standards Board's Accounting Standards Codification (“ASC”) 718 “Compensation-Stock Compensation” the MYLTIP awards have an aggregate value of approximately $8.1 million, which amount will generally be amortized into earnings over the five-year plan period under the graded vesting method.

On January 29, 2013, the Company entered into an agreement to acquire a parcel of land located in Reston, Virginia for a purchase price of approximately $27.0 million in cash. There can be no assurance that the acquisition will be completed on the terms currently contemplated or at all.

On February 1, 2013, the Company issued 35,087 shares of restricted common stock, 153,006 LTIP units and 201,373 non-qualified stock options under the 2012 Plan to certain employees of the Company.

On February 5, 2013, the Company used available cash to repay the mortgage loan collateralized by its Kingstowne One property located in Alexandria, Virginia totaling approximately $17.0 million. The mortgage loan bore interest at a fixed rate of 5.96% per annum and was scheduled to mature on May 5, 2013. There was no prepayment penalty.

On February 6, 2013, the Company completed the acquisition of 535 Mission Street, a development site, in San Francisco, California for an aggregate purchase price of approximately $71.0 million in cash, including work completed and materials purchased to date. When completed, 535 Mission Street will consist of a 27-story, Class A office tower with approximately 307,000 net rentable square feet of office and retail space. The Company has commenced development of the project.

On February 7, 2013, the partner in the Company's joint venture that is pursuing the acquisition of land in San Francisco, California which could support the office tower known as Transbay Tower, issued a notice that it has elected under the joint venture agreement to reduce its nominal ownership interest in the venture from 50% to 5%. On February 26, 2013, the Company issued a notice to such partner electing to proceed with the venture on that basis. As a result, the Company has a 95% nominal interest in, and expects to consolidate, the joint venture.