Acquisitions (Details) (USD $)	6 Months Ended
	Jun. 30, 2011
Acquired operating properties from unrelated third parties
Number of Buildings	8
Rentable Square Feet	1,160,233
Purchase Price	$ 413,000,000
Assets
Buildings and improvements	357,640,000
Land	36,740,000
Undeveloped land	2,560,000
Deferred leasing costs and acquisition-related intangible assets	31,290,000
Total assets acquired	428,230,000
Liabilities
Deferred revenue and acquisition-related intangible liabilities	14,240,000
Secured debt	30,997,000
Accounts payable, accrued expenses and other liabilities	4,439,000
Total liabilities assumed	49,676,000
Net assets and liabilities acquired	378,554,000
Office Properties [Member]
Acquired operating properties from unrelated third parties
Number of Buildings	8
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	90,742
Percentage Occupied	76.70%
Purchase Price	33,000,000
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
Number of Buildings	4
Rentable Square Feet	279,924
Percentage Occupied	87.30%
Purchase Price	100,100,000
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
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	89.80%
Purchase Price	215,000,000
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
601 108th Avenue N.E Bellevue, WA [Member]
Assets
Buildings and improvements	214,095,000
Land	0
Undeveloped land	0
Deferred leasing costs and acquisition-related intangible assets	13,790,000
Total assets acquired	227,885,000
Liabilities
Deferred revenue and acquisition-related intangible liabilities	12,850,000
Secured debt	0
Accounts payable, accrued expenses and other liabilities	2,380,000
Total liabilities assumed	15,230,000
Net assets and liabilities acquired	212,655,000
All Other Acquisitions [Member]
Assets
Buildings and improvements	143,545,000
Land	36,740,000
Undeveloped land	2,560,000
Deferred leasing costs and acquisition-related intangible assets	17,500,000
Total assets acquired	200,345,000
Liabilities
Deferred revenue and acquisition-related intangible liabilities	1,390,000
Secured debt	30,997,000
Accounts payable, accrued expenses and other liabilities	2,059,000
Total liabilities assumed	34,446,000
Net assets and liabilities acquired	$ 165,899,000