Real Estate Properties - Summary of Properties Held for Sale (Details) $ in Millions	1 Months Ended
	Jul. 31, 2017 USD ($) ft² building	Jun. 30, 2017 ft² property building	Dec. 31, 2016 property
Real Estate Properties [Line Items]
Number of Properties			0
Held-for-sale
Real Estate Properties [Line Items]
Number of Properties		6
Number of Buildings | building		8
Square Footage (in sqft) | ft²		2,761,288
Held-for-sale | 1500 Market Street
Real Estate Properties [Line Items]
Number of Properties		1
Held-for-sale | 820 W. Diamond
Real Estate Properties [Line Items]
Number of Properties		1
Number of Buildings | building		1
Square Footage (in sqft) | ft²		134,933
Held-for-sale | Danac Stiles Business Park
Real Estate Properties [Line Items]
Number of Properties		1
Number of Buildings | building		3
Square Footage (in sqft) | ft²		276,637
Held-for-sale | 411 Farwell Avenue
Real Estate Properties [Line Items]
Number of Properties		1
Number of Buildings | building		1
Square Footage (in sqft) | ft²		422,727
Held-for-sale | 2250 Pilot Knob Road
Real Estate Properties [Line Items]
Number of Properties		1
Number of Buildings | building		1
Square Footage (in sqft) | ft²		87,183
Held-for-sale | 4700 Belleview Avenue
Real Estate Properties [Line Items]
Number of Properties		1
Number of Buildings | building		1
Square Footage (in sqft) | ft²		80,615
Held-for-sale | Five Property Portfolio
Real Estate Properties [Line Items]
Number of Properties		5
Number of Buildings | building		7
Square Footage (in sqft) | ft²		1,002,095
Subsequent event | Held-for-sale | 1500 Market Street
Real Estate Properties [Line Items]
Number of Buildings | building	1
Square footage (in sqft) | ft²	1,759,193
Gross Sales Price | $	$ 328.0