Acquisitions (Details) (USD $)	9 Months Ended
	Sep. 30, 2011 NumberofTradingDays Years RSF NumberofBuildings Share_Based_Incentive_Compensation_Plans
Acquired operating properties from unrelated third parties
Number of Buildings	9
Rentable Square Feet	1,473,984
Purchase Price	$ 516,300,000	[1]
Assets
Land	56,000,000
Buildings and improvements	441,658,000	[2]
Undeveloped Land	2,560,000
Deferred leasing costs and acquisition-related intangible assets	39,990,000	[3]
Total Assets Acquired	540,208,000
Liabilities
Deferred revenue and acquisition-related intangible liabilities	22,940,000	[4]
Secured debt	30,997,000	[5]
Accounts payable, accrued expenses, and other liabilities	4,515,000
Total liabilities assumed	58,452,000
Net assets and liabilities acquired	481,756,000	[6]
Office Properties [Member]
Acquired operating properties from unrelated third parties
Number of Buildings	9
Office Properties [Member] | 601 108th Avenue N.E Bellevue, WA [Member]
Acquired operating properties from unrelated third parties
Date of Acquisition	June 3, 2011
Number of Buildings	1
Rentable Square Feet	488,470
Percentage Occupied	88.10%
Purchase Price	215,000,000	[1]
Office Properties [Member] | 201 Third Street San Francisco CA [Member]
Acquired operating properties from unrelated third parties
Date of Acquisition	September 15, 2011	[7]
Number of Buildings	1	[7]
Rentable Square Feet	311,545	[7]
Percentage Occupied	90.30%	[7]
Purchase Price	103,300,000	[1],[7]
Office Properties [Member] | 250 Brannan Street San Francisco, CA [Member]
Acquired operating properties from unrelated third parties
Date of Acquisition	January 28, 2011
Number of Buildings	1
Rentable Square Feet	92,948
Percentage Occupied	100.00%
Purchase Price	33,000,000	[1]
Office Properties [Member] | 10210, 10220, and 1023 NE Points Drive; 3933 Lake Washington Boulevard NE Kirkland, WA [Member]
Acquired operating properties from unrelated third parties
Date of Acquisition	April 21, 2011	[8]
Number of Buildings	4	[8]
Rentable Square Feet	279,924	[8]
Percentage Occupied	89.60%	[8]
Purchase Price	100,100,000	[1],[8]
Office Properties [Member] | 10770 Wateridge Circle San Diego, CA [Member]
Acquired operating properties from unrelated third parties
Date of Acquisition	May 12, 2011
Number of Buildings	1
Rentable Square Feet	174,310
Percentage Occupied	97.50%
Purchase Price	32,700,000	[1]
Office Properties [Member] | 4040 Civic Center Drive San Rafael, CA [Member]
Acquired operating properties from unrelated third parties
Date of Acquisition	June 9, 2011
Number of Buildings	1
Rentable Square Feet	126,787
Percentage Occupied	93.10%
Purchase Price	32,200,000	[1]
601 108th Avenue N.E Bellevue, WA [Member]
Assets
Land	0	[10],[9]
Buildings and improvements	214,095,000	[10],[2]
Undeveloped Land	0	[10]
Deferred leasing costs and acquisition-related intangible assets	13,790,000	[10]
Total Assets Acquired	227,885,000
Liabilities
Deferred revenue and acquisition-related intangible liabilities	12,850,000	[10]
Secured debt	0	[10]
Accounts payable, accrued expenses, and other liabilities	2,380,000	[10]
Total liabilities assumed	15,230,000	[10]
Net assets and liabilities acquired	212,655,000	[10],[6]
201 Third Street San Francisco CA [Member]
Assets
Land	19,260,000	[10]
Buildings and improvements	84,018,000	[10],[2]
Undeveloped Land	0	[10]
Deferred leasing costs and acquisition-related intangible assets	8,700,000	[10]
Total Assets Acquired	111,978,000	[10]
Liabilities
Deferred revenue and acquisition-related intangible liabilities	8,700,000	[10]
Secured debt	0	[10]
Accounts payable, accrued expenses, and other liabilities	76,000	[10]
Total liabilities assumed	8,776,000	[10]
Net assets and liabilities acquired	103,202,000	[10],[6]
All Other Acquisitions [Member]
Assets
Land	36,740,000	[11]
Buildings and improvements	143,545,000	[11],[2]
Undeveloped Land	2,560,000	[11]
Deferred leasing costs and acquisition-related intangible assets	17,500,000	[11]
Total Assets Acquired	200,345,000
Liabilities
Deferred revenue and acquisition-related intangible liabilities	1,390,000	[11]
Secured debt	30,997,000	[11],[5]
Accounts payable, accrued expenses, and other liabilities	2,059,000	[11]
Total liabilities assumed	34,446,000	[11]
Net assets and liabilities acquired	$ 165,899,000	[11],[6]

[1] Excludes acquisition-related costs.

[2] Represents buildings, building improvements and tenant improvements.

[3] Represents in-place leases (approximately $25.2 million with a weighted average amortization period of 3.9 years), above-market leases (approximately $6.8 million with a weighted average amortization period of 4.5 years), and leasing commissions (approximately $7.9 million with a weighted average amortization period of 2.6 years).

[4] Represents below-market leases (approximately $17.7 million with a weighted average amortization period of 3.8 years) and an above-market ground lease obligation (approximately $5.2 million with a weighted average amortization period of 82.5 years), under which we are the lessee.

[5] Represents the mortgage loan, which includes an unamortized premium recorded at the date of acquisition of approximately $1.0 million in connection with recording this debt at fair value, assumed in connection with the properties acquired in April 2011 (see Note 5).

[6] Reflects the purchase price net of assumed secured debt and other lease-related obligations.

[7] As of September 30, 2011, this property was temporarily being held in a separate VIE to facilitate a potential Section 1031 Exchange (see Note 1).

[8] In connection with this acquisition, we assumed secured debt with an outstanding principal balance of $30.0 million and recorded a premium of $1.0 million in connection with recording this debt at fair value on the acquisition date (see Note 5).

[9] In connection with the acquisition of 601 108th Avenue N.E., Bellevue, WA, we assumed the lessee's obligations under a noncancellable ground lease that is scheduled to expire in November 2093 (see Notes 3 and 11).

[10] The purchase of 601 108th Avenue N.E., Bellevue, WA, and 201 Third Street, San Francisco, CA, represent the two largest acquisitions and 61.7% of the total aggregate purchase price of the properties acquired during the nine months ended September 30, 2011.

[11] The purchase price of all other acquisitions completed during the nine months ended September 30, 2011 were individually less than 5% and in the aggregate less than 10% of the Company's total assets as of December 31, 2010.