Dispositions - Operating Property Dispositions (Details) $ in Thousands	1 Months Ended				12 Months Ended
	Oct. 31, 2019 USD ($) ft² property	May 31, 2019 USD ($) ft² property	Dec. 31, 2018 USD ($) ft² property	Nov. 30, 2018 USD ($) ft² property	Dec. 31, 2020 USD ($) ft² property	Dec. 31, 2019 USD ($) ft² property	Dec. 31, 2018 USD ($) ft² property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²					14,620,166
Gains on sales of operating properties | $					$ 35,536	$ 36,802	$ 142,926
Disposal group, disposed of by sale [Member] | 2020 Dispositions [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings | property					1
Rentable square feet | ft²					87,147
Proceeds from sale of property held-for-sale | $					$ 75,900
Disposal group, disposed of by sale [Member] | 331 Fairchild Drive, Mountain View, CA [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings | property					1
Rentable square feet | ft²					87,147
Proceeds from sale of property held-for-sale | $					$ 75,900
Disposal group, disposed of by sale [Member] | 2019 Dispositions [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings | property						2
Rentable square feet | ft²						355,654
Proceeds from sale of property held-for-sale | $						$ 133,800
Disposal group, disposed of by sale [Member] | 2829 Townsgate Road, Thousand Oaks, CA [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings | property		1
Rentable square feet | ft²		84,098
Proceeds from sale of property held-for-sale | $		$ 18,300
Disposal group, disposed of by sale [Member] | 2211 Michelson Drive Irvine CA [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings | property	1
Rentable square feet | ft²	271,556
Proceeds from sale of property held-for-sale | $	$ 115,500
Disposal group, disposed of by sale [Member] | 2018 Dispositions [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings | property			11				11
Rentable square feet | ft²			772,246				772,246
Proceeds from sale of property held-for-sale | $							$ 373,000
Disposal group, disposed of by sale [Member] | 1325-1327 Chesapeake Terrace, Sunnyvale, CA [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings | property				4
Rentable square feet | ft²				266,982
Proceeds from sale of property held-for-sale | $				$ 160,300
Disposal group, disposed of by sale [Member] | Plaza Yarrow Bay Properties [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings | property				4
Rentable square feet | ft²				279,924
Proceeds from sale of property held-for-sale | $				$ 134,500
Disposal group, disposed of by sale [Member] | 23925, 23975, & 24025 Park Sorrento, Calabasas, CA [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of buildings | property			3				3
Rentable square feet | ft²			225,340				225,340
Proceeds from sale of property held-for-sale | $			$ 78,200