Building Acquisitions and Dispositions (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) ft² a bldg	Mar. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Gross Proceeds	$ 184,233	$ 1,874
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	24
Leaseable square feet | ft²	1,500,515
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross Proceeds	$ 193,616
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	9.8
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1
Leaseable square feet | ft²	80,000
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross Proceeds	$ 7,094
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	1.5
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0
Leaseable square feet | ft²	0
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross Proceeds	$ 2,714
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	8.3
Disposal Group, Not Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	23
Leaseable square feet | ft²	1,420,515
Disposal Group, Not Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross Proceeds	$ 183,808
Disposal Group, Not Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0
2018 acquisitions current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1
Leaseable square feet | ft²	400,169
2018 acquisitions current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase Price	$ 96,940
2018 acquisitions current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	7.9
2018 acquisitions current quarter [Member] | United Kingdom [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0
Leaseable square feet | ft²	0
2018 acquisitions current quarter [Member] | United Kingdom [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase Price	$ 4,240
2018 acquisitions current quarter [Member] | United Kingdom [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	7.1
2018 acquisitions current quarter [Member] | Southern California [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1
Leaseable square feet | ft²	400,169
2018 acquisitions current quarter [Member] | Southern California [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase Price	$ 92,700
2018 acquisitions current quarter [Member] | Southern California [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0.8