Investment in Real Estate - Summary of Property Sold (Details) $ in Millions					9 Months Ended
	Jun. 01, 2016 USD ($) ft² building	Apr. 07, 2016 USD ($) ft² building	Jan. 14, 2016 USD ($) ft² building	Mar. 06, 2015 USD ($) ft² building	Sep. 30, 2016 USD ($) ft² building	Sep. 30, 2015 USD ($) ft² building	Jan. 07, 2015
Real Estate [Line Items]
Number of real estate properties acquired \| building					6	2
Area of real estate property (in square feet) \| ft²					820,587	483,862
Proceeds from sale of real estate \| $					$ 287.4	$ 179.0
Bayhill Office Center
Real Estate [Line Items]
Number of real estate properties acquired \| building			4
Area of real estate property (in square feet) \| ft²			554,328
Proceeds from sale of real estate \| $			$ 215.0
Patrick Henry Drive
Real Estate [Line Items]
Number of real estate properties acquired \| building		1
Area of real estate property (in square feet) \| ft²		70,520
Proceeds from sale of real estate \| $		$ 19.0
One Bay Plaza
Real Estate [Line Items]
Number of real estate properties acquired \| building	1
Area of real estate property (in square feet) \| ft²	195,739
Proceeds from sale of real estate \| $	$ 53.4
First Financial
Real Estate [Line Items]
Number of real estate properties acquired \| building				1
Area of real estate property (in square feet) \| ft²				223,679
Proceeds from sale of real estate \| $				$ 89.0
Bay Park Plaza
Real Estate [Line Items]
Number of real estate properties acquired \| building				1
Area of real estate property (in square feet) \| ft²				260,183
Proceeds from sale of real estate \| $				$ 90.0
1455 Market Street
Real Estate [Line Items]
Real estate property, ownership (as a percent)							45.00%