Investment in Real Estate - Properties Disposed (Details) $ in Thousands					3 Months Ended		6 Months Ended
	Apr. 10, 2018 USD ($) ft²	Mar. 01, 2018 USD ($) ft²	Jan. 31, 2018 USD ($) ft²	Jan. 25, 2018 USD ($) ft²	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) ft²	Jun. 30, 2017 USD ($)
Real Estate [Line Items]
Square feet of real estate disposed \| ft²							367,487
Sales Price							$ 254,800
Gains on sale of real estate					$ 1,928	$ 0	$ 39,602	$ 16,866
Embarcadero Place
Real Estate [Line Items]
Square feet of real estate disposed \| ft²				197,402
Sales Price				$ 136,000
2600 Campus Drive (building 6 of Peninsula Office Park)
Real Estate [Line Items]
Square feet of real estate disposed \| ft²			63,050
Sales Price			$ 22,500
2180 Sand Hill
Real Estate [Line Items]
Square feet of real estate disposed \| ft²		45,613
Sales Price		$ 82,500
9300 Wilshire
Real Estate [Line Items]
Square feet of real estate disposed \| ft²	61,422
Sales Price	$ 13,800