Investment in Real Estate - Schedule of Dispositions (Details) $ in Millions													12 Months Ended
	Jul. 27, 2018 USD ($) ft²	Apr. 10, 2018 USD ($) ft²	Mar. 01, 2018 USD ($) ft²	Jan. 31, 2018 USD ($) ft²	Jan. 25, 2018 USD ($) ft²	Nov. 16, 2017 USD ($) ft²	Mar. 21, 2017 USD ($) ft²	Feb. 14, 2017 USD ($) ft²	Nov. 04, 2016 USD ($) ft²	Jun. 01, 2016 USD ($) ft²	Apr. 07, 2016 USD ($) ft²	Jan. 14, 2016 USD ($) ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($) ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²													815,226	823,261	921,343
Sales Price | $													$ 464.8	$ 436.8	$ 367.4
Embarcadero Place
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²					197,402
Sales Price | $					$ 136.0
2600 Campus Drive
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²				63,050
Sales Price | $				$ 22.5
2180 Sand Hill Road
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²			45,613
Sales Price | $			$ 82.5
9300 Wilshire
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²		61,422
Sales Price | $		$ 13.8
Peninsula Office Park
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²	447,739
Sales Price | $	$ 210.0
222 Kearny Street
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²								148,797
Sales Price | $								$ 51.8
3402 Pico
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²							50,687
Sales Price | $							$ 35.0
Pinnacle I and II
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²						623,777
Sales Price | $						$ 350.0
Bayhill Office Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²												554,328
Sales Price | $												$ 215.0
Patrick Henry
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²											70,520
Sales Price | $											$ 19.0
One Bay Plaza
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²										195,739
Sales Price | $										$ 53.4
12655 Jefferson
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of real estate property | ft²									100,756
Sales Price | $									$ 80.0