Building Acquisitions and Dispositions (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) ft² bldg	Mar. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Buildings and Land	$ 1,874	$ 127,727
Disposal Group, Not Discontinued Operations [Member] | 2017 sales [Member] | Operating Segments [Member] | Southeastern PA [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1
Net Rentable Area | ft²	33,000
Disposal Group, Not Discontinued Operations [Member] | 2017 sales [Member] | Operating Segments [Member] | Southeastern PA [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Buildings and Land	$ 2,100