Investment in Real Estate - Properties Disposed (Details) $ in Thousands						3 Months Ended		9 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 ($) ft²	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) ft²	Sep. 30, 2017 USD ($)
Real Estate [Line Items]
Square feet of real estate disposed \| ft²								815,226
Sales Price								$ 464,800
Gains on sale of real estate						$ 3,735	$ 0	$ 43,337	$ 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
Peninsula Office Park
Real Estate [Line Items]
Square feet of real estate disposed \| ft²	447,739
Sales Price	$ 210,000