Real Estate Assets Held for Sale and Dispositions (Details) (USD $)	3 Months Ended					0 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014	Jan. 15, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of land (Note 3)	$ 17,268,000		$ 0
Proceeds from Sale of Real Estate Held-for-investment	25,563,000		309,824,000
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Disposal Group, Including Discontinued Operation, Land and improvements	38,706,000
Disposal Group, Including Discontinued Operation, Buildings and Improvements	198,125,000
Disposal Group, Including Discontinued Operation, Total real estate held-for-sale	236,831,000
Disposal Group, Including Discontinued Operation, Accumulated depreciation and amortization	66,421,000
Disposal Group, Including Discontinued Operation, Total real estate held for sale, net	170,410,000
Disposal Group, Including Discontinued Operation, Current receivables, net	185,000
Disposal Group, Including Discontinued Operation, Deferred rent receivables, net	7,285,000
Disposal Group, Including Discontinued Operation, Deferred leasing costs and acquisition-related intangible assets, net	12,364,000
Disposal Group, Including Discontinued Operation, Prepaid expenses and other assets, net	507,000
Disposal Group, Including Discontinued Operation, Real estate and other assets held for sale, net	190,751,000				8,211,000
Disposal Group, Including Discontinued Operation, Accounts payable, accrued expenses and other liabilities	7,376,000
Disposal Group, Including Discontinued Operation, Deferred revenue and acquisition-related intangible liabilities, net	1,367,000
Disposal Group, Including Discontinued Operation, Rents received in advance and tenant security deposits	1,025,000
Disposal Group, Including Discontinued Operations, Liabilities and deferred revenue of real estate assets held for sale	9,768,000				56,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income from discontinued operations	0		943,000
Gains on dispositions of discontinued operations	0		90,115,000
Total income from discontinued operations	0	[1]	91,058,000	[1]
Redmond, WA Operating Property [Member] [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings	1
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net Rentable Area	122,103
San Diego Portfolio [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings	9	[2]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net Rentable Area	922,741	[2]
Restricted cash [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Real Estate Held-for-investment					59,200,000
Properties Held for Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings	10
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net Rentable Area	1,044,844
Properties Held for Sale [Member] | Von Karman, Irvine, California [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Dispositions Sales Price						$ 26,000,000

[1] The Company adopted ASU 2014-08 effective January 1, 2015 (see Note 1). As a result, properties classified as held for sale and/or disposed of subsequent to January 1, 2015 that do not represent a strategic shift are no longer presented as discontinued operations.

[2] The San Diego Properties include the following: 6260 Sequence Drive, 6290 Sequence Drive, 6310 Sequence Drive, 6340 Sequence Drive, 6350 Sequence Drive, 10770 Wateridge Circle, 4921 Directors Place, 6200 Greenwich Drive, and 6220 Greenwich Drive. The properties are being sold in two tranches. The Company completed the sale of the first tranche for gross proceeds of approximately $95.0 million on April 15, 2015.