Acquisitions (Details) (USD $)									12 Months Ended								12 Months Ended																12 Months Ended			12 Months Ended		12 Months Ended			12 Months Ended		12 Months Ended		12 Months Ended									12 Months Ended
	Dec. 31, 2012 sqft		Dec. 31, 2011		Dec. 31, 2012 Real Estate Acquisitions [Member] building sqft		Dec. 31, 2011 Real Estate Acquisitions [Member] building sqft		Dec. 31, 2012 Bohannon Drive Menlo Park, CA [Member] sqft building		Dec. 31, 2012 Fremont Lake Union Center Seattle WA [Member] sqft building		Dec. 31, 2012 Fremont Lake View Seattle WA [Member] building sqft		Dec. 31, 2012 Fourth Street Bellevue WA [Member]		Dec. 31, 2012 Olympic Blvd 12233, Los Angeles, CA [Member] building sqft		Dec. 31, 2012 N. Mathilda Avenue 599, Sunnyvale, CA [Member] sqft building		Dec. 31, 2011 601 108th Avenue N.E Bellevue, WA [Member]		Dec. 31, 2011 201 Third Street San Francisco CA [Member]		Dec. 31, 2012 All Other Acquisitions [Member]		Dec. 31, 2011 All Other Acquisitions [Member]		Dec. 31, 2012 Development and Redevelopment Project Acquisitions [Member] project		Dec. 31, 2011 Development and Redevelopment Project Acquisitions [Member]		Dec. 31, 2012 Middlefield, Mountain View, CA [Member]	May 09, 2012 Middlefield, Mountain View, CA [Member]		Dec. 31, 2012 Brannan Street 333, San Francisco, CA [Member]	Oct. 23, 2012 Brannan Street 333, San Francisco, CA [Member]	Dec. 31, 2012 6121 Sunset Boulevard, Los Angeles, CA [Member]	Sep. 28, 2012 6121 Sunset Boulevard, Los Angeles, CA [Member]		Dec. 31, 2012 Mission St., 350, San Francisco, CA [Member]	Oct. 23, 2012 Mission St., 350, San Francisco, CA [Member]	Dec. 31, 2012 Fairchild Drive, 331, Mountain View, CA [Member]	Dec. 04, 2012 Fairchild Drive, 331, Mountain View, CA [Member]	Dec. 31, 2012 Mathilda, 555, Sunnyvale, CA [Member]		Dec. 10, 2012 Mathilda, 555, Sunnyvale, CA [Member]		Dec. 31, 2012 All Other Acquisitions [Member]		Dec. 31, 2012 Office Properties [Member] sqft		Dec. 31, 2012 Office Properties [Member] Real Estate Acquisitions [Member] building	Dec. 31, 2012 Office Properties [Member] Fourth Street Bellevue WA [Member] building sqft		Dec. 31, 2012 Office Properties [Member] 6255 Sunset Boulevard, Los Angeles, CA [Member] building sqft		Dec. 31, 2011 Office Properties [Member] 250 Brannan Street San Francisco, CA [Member] building sqft		Dec. 31, 2011 Office Properties [Member] 10210, 10220, and 1023 NE Points Drive; 3933 Lake Washington Boulevard NE Kirkland, WA [Member] building sqft		Dec. 31, 2011 Office Properties [Member] 10770 Wateridge Circle San Diego, CA [Member] sqft building		Dec. 31, 2011 Office Properties [Member] 601 108th Avenue N.E Bellevue, WA [Member] sqft building		Dec. 31, 2011 Office Properties [Member] 4040 Civic Center Drive San Rafael, CA [Member] sqft building		Dec. 31, 2011 Office Properties [Member] 201 Third Street San Francisco CA [Member] building sqft		Dec. 31, 2011 Office Properties [Member] 301 Brannan Street San Francisco CA [Member] sqft building		Dec. 31, 2011 Office Properties [Member] 370 Third Street, San Francisco, CA [Member]
Acquired operating properties from unrelated third parties
Date of Acquisition									Feb. 29, 2012		Jun. 01, 2012	[1]	Jun. 01, 2012				Oct. 05, 2012	[2]	Dec. 17, 2012	[3]													May 09, 2012			Jul. 20, 2012		Sep. 28, 2012			Oct. 23, 2012		Dec. 04, 2012		Dec. 17, 2012									Jul. 24, 2012	[4],[5]	Jul. 31, 2012	[6]	Jan. 28, 2011		Apr. 21, 2011	[7]	May 12, 2011		Jun. 03, 2011		Jun. 09, 2011		Sep. 15, 2011		Nov. 15, 2011		Dec. 15, 2011
Number of buildings					14		10		7		2	[1]	1				1	[2]	1	[3]									7																								24	1	[4],[5]	1	[6]	1		4	[7]	1		1		1		1		1
Rentable square feet	13,249,780	[8]			1,759,603		1,568,945		374,139		308,407	[1]	111,580				151,029	[2]	75,810	[3]																															13,249,780	[8]		416,755	[4],[5]	321,883	[6]	92,948		279,924	[7]	174,310		488,470		126,787		332,076		74,430
Percentage Occupied									84.70%		99.40%	[1]	100.00%				96.80%	[2]	100.00%	[3]																																		90.50%	[4],[5]	85.20%	[6]	100.00%		90.00%	[7]	97.50%		90.40%		98.10%		99.50%		100.00%
Purchase Price					$ 674,000,000	[9]	$ 546,300,000	[9]	$ 162,500,000	[9]	$ 105,400,000	[1],[9]	$ 39,200,000	[9]			$ 72,900,000	[2],[9]	$ 29,100,000	[3],[9]									$ 340,300,000		$ 91,500,000	[9]		$ 74,500,000	[10]		$ 18,500,000		$ 65,000,000	[11]		$ 52,000,000		$ 21,800,000			$ 108,500,000	[12]						$ 186,100,000	[4],[5],[9]	$ 78,800,000	[6],[9]	$ 33,000,000	[9]	$ 100,100,000	[7],[9]	$ 32,700,000	[9]	$ 215,000,000	[9]	$ 32,200,000	[9]	$ 103,300,000	[9]	$ 30,000,000	[9]	$ 91,500,000	[9]
Assets
Land	117,639,000	[13]	61,910,000						38,810,000	[13]					25,080,000	[13]			13,538,000	[13]			19,260,000		40,211,000	[13],[14]	42,650,000	[14]
Buildings and improvements	545,510,000	[15]	464,108,000	[15]					124,617,000	[15]					150,877,000	[15]			12,558,000	[15]	214,095,000	[15]	84,018,000	[15]	257,458,000	[14],[15]	165,995,000	[14],[15]
Undeveloped land and construction in progress			2,560,000	[16]																	0		0				2,560,000	[14],[16]	364,827,000		89,345,000														120,243,000				244,584,000
Cash and cash equivalents	3,973,000								0		4,000,000				0				0						3,973,000	[14]
Restricted cash	5,329,000								0						0				0						5,329,000	[14]			11,250,000																11,250,000	[17]			0
Deferred leasing costs and acquisition-related intangible assets	59,513,000	[18]	42,630,000	[19]					9,470,000	[18]					16,469,000	[18]			3,004,000	[18]	13,790,000	[19]	8,700,000	[19]	30,570,000	[14],[18]	20,140,000	[14],[19]			2,930,000
Prepaid expenses and other assets	184,000								0						0				0						184,000	[14]			1,300,000																0				1,300,000
Total Assets Acquired	732,148,000		571,208,000						172,897,000						192,426,000				29,100,000		227,885,000		111,978,000		337,725,000	[14]	231,345,000	[14]	377,377,000		92,275,000														131,493,000				245,884,000
Liabilities
Deferred revenue and acquisition-related intangible liabilities	35,020,000	[20]	23,940,000	[21]					10,380,000	[20]					4,940,000	[20]			0	[20]	12,850,000	[21]	8,700,000	[21]	19,700,000	[14],[20]	2,390,000	[14],[21]			730,000
Secured debt,net	221,032,000	[22]	30,997,000	[23]					0	[22]					84,984,000	[22]			0	[22]	0	[23]	0	[23]	136,048,000	[14],[22]	30,997,000	[14],[23]
Accounts payable, accrued expenses, and other liabilities	6,348,000		4,515,000						137,000		600,000				627,000				0		2,380,000		76,000		5,584,000	[14]	2,059,000	[14]	32,823,000																23,071,000	[17]			9,752,000
Total liabilities assumed	262,400,000		59,452,000						10,517,000						90,551,000				0		15,230,000		8,776,000		161,332,000	[14]	35,446,000	[14]	32,823,000		730,000														23,071,000				9,752,000
Net assets and liabilities acquired	$ 469,748,000	[24]	$ 511,756,000						$ 162,380,000	[24]					$ 101,875,000	[24]			$ 29,100,000	[24]	$ 212,655,000		$ 103,202,000		$ 176,393,000	[14],[24]	$ 195,899,000	[14]	$ 344,554,000	[25]	$ 91,545,000														$ 108,422,000	[25]			$ 236,132,000	[25]

[1] We acquired these properties through the acquisition of the ownership interest of the bankruptcy remote LLC that owned the properties. In connection with this acquisition we also acquired cash of approximately $4.0 million and other assets of approximately $0.2 million and we assumed current liabilities of approximately $0.6 million and secured debt with an outstanding principal balance of $34.0 million and a premium of $1.7 million as a result of recording the debt at fair value at the acquisition date (see Note 7).

[2] In connection with this acquisition, we assumed secured debt with an outstanding principal balance of $40.7 million and a premium of $2.7 million as a result of recording this debt at fair value on the acquisition date (see Note 7).

[3] This operating property was acquired in connection with the purchase of the 555 N. Mathilda Ave. development property discussed in further detail in the "Development and Redevelopment Project Site" section of this footnote, for a total purchase price of $137.6 million. The acquisition of both the operating property and the development site in a single transaction constituted our third largest acquisition of 2012.

[4] In connection with this acquisition, we assumed secured debt with an outstanding principal balance of $83.6 million and a premium of $1.4 million as a result of recordingthis debt at fair value on the acquisition date. In January 2013, we repaid this loan prior to the stated maturity (see Note 7).

[5] As of December 31, 2012, these properties were temporarily being held in separate VIEs to facilitate potential Section 1031 Exchanges (see Note 2). The VIE was terminated subsequent to year-end.

[6] As part of the consideration for this transaction, we issued 118,372 common units of the Operating Partnership valued at $47.34 per unit, which was the Company's closing stock price on the NYSE on the acquisition date. In connection with this acquisition we also assumed secured debt with an outstanding principal balance of $53.9 million and a premium of $3.1 million as a result of recording this debt at fair value on the acquisition date (see Note 7). We also assumed $4.7 million of accrued liabilities in connection with this acquisition that are not included in the purchase price above.

[7] In connection with this acquisition, we assumed secured debt with an outstanding principal balance of $30.0 million and an initial premium of $1.0 million as a result of recording this debt at fair value on the acquisition date (see Note 7).

[8] Includes square footage from our stabilized portfolio.

[9] Excludes acquisition-related costs and non-lease related accrued liabilities assumed. Includes assumed unpaid leasing commissions and tenant improvements.

[10] The total purchase price for this acquisition was comprised of a cash purchase price of $74.5 million plus $9.5 million of assumed leasing commissions and other accrued liabilities

[11] In connection with this acquisition we also assumed $1.1 million of other accrued liabilities which are not included in the purchase price above.

[12] As of December 31, 2012, this property was temporarily being held in a separate VIE to facilitate a potential Section 1031 Exchange (see Note 2).

[13] In connection with the acquisitions of 701, 801, and 837 N. 34th Street, Seattle, WA, we assumed the lessee obligations under a ground lease with an initial expiration in December 2041. The ground lease obligation contains three 10-year extension options and one 45-year extension option. In connection with the acquisitions of 601 108th Avenue N.E., Bellevue, WA, we assumed the lessee obligation under a ground lease that is scheduled to expire in November 2093 (see Note 15 for additional information pertaining to these ground leases).

[14] The purchase price of all other acquisitions during the years ended December 31, 2012 and 2011 were individually less than 5% and in aggregate less than 10% of the Company's total assets as of December 31, 2012 and 2011, respectively.

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

[16] Represents in-place leases (approximately $27.4 million with a weighted average amortization period of 3.8 years), above-market leases (approximately $6.8 million with a weighted average amortization period of 4.5 years years) and unamortized leasing commissions (approximately $8.5 million with a weighted average amortization period of 2.5 years)

[17] In connection with this acquisition, restricted cash is being held in escrow to pay for potential environmental costs and contingent development costs. Any unused amounts will be released to the seller.

[18] Represents in-place leases (approximately $43.4 million with a weighted average amortization period of 4.7 years), above-market leases (approximately $1.4 million with a weighted average amortization period of 3.8 years), leasing commissions (approximately $14.2 million with a weighted average amortization period of 3.4 years), and a below-market ground lease obligation (approximately $0.5 million with a weighted average amortization period of 59.6 years).

[19] Represents below-market leases (approximately $18.7 million with a weighted average amortization period of 3.9 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.

[20] Represents below-market leases (approximately $33.9 million with a weighted average amortization period of 6.5 years) and an above-market ground lease obligation (approximately $1.1 million with a weighted average amortization period of 29.6 years).

[21] Represents the mortgage loan, which includes an unamortized premium of approximately $1.0 million at the date of acquisition, assumed in connection with the properties acquired in April 2011 (see Note 7).

[22] Represents the fair value of the mortgage loans assumed, which includes an aggregate unamortized premium balance of approximately $8.9 million at the dates of acquisition (see Note 7).

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

[24] Reflects the purchase price plus cash and restricted cash received, net of assumed secured debt, lease-related obligations and other accrued liabilities.

[25] Reflects the purchase price including assumed leasing commissions, net of assumed accrued liabilities.