Real Estate (Real Estate Properties Acquired) (Details) $ in Thousands	2 Months Ended	12 Months Ended
	Feb. 27, 2018 USD ($)	Dec. 31, 2017 USD ($) ft² a bldg	Dec. 31, 2016 USD ($) ft² a bldg	Dec. 31, 2015 USD ($) ft² a bldg	Jan. 01, 2018 ft²
Business Acquisition [Line Items]
Number of buildings | bldg		37
Acres of developable land | a		9
Operating Segments [Member] | Building and Building Improvements [Member] | United Kingdom [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		0
Leaseable square feet | ft²		0
Operating Segments [Member] | Land and Land Improvements [Member] | Chicago Minneapolis [Member]
Business Acquisition [Line Items]
Acres of developable land | a			6
Operating Segments [Member] | Land and Land Improvements [Member] | Philadelphia [Member]
Business Acquisition [Line Items]
Acres of developable land | a			6
Operating Segments [Member] | Land and Land Improvements [Member] | Southeastern PA [Member]
Business Acquisition [Line Items]
Acres of developable land | a			8
Operating Segments [Member] | Land and Land Improvements [Member] | United Kingdom [Member]
Business Acquisition [Line Items]
Acres of developable land | a		35
2015 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Business Acquisition [Line Items]
Number of buildings | bldg				5
Leaseable square feet | ft²				1,529,211
2015 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Number of buildings | bldg				0
Leaseable square feet | ft²				0
2015 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Chicago Minneapolis [Member]
Business Acquisition [Line Items]
Number of buildings | bldg				1
Leaseable square feet | ft²				197,956
2015 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Number of buildings | bldg				0
Leaseable square feet | ft²				0
2015 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Number of buildings | bldg				1
Leaseable square feet | ft²				410,059
2015 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Houston [Member]
Business Acquisition [Line Items]
Number of buildings | bldg				3
Leaseable square feet | ft²				921,196
2015 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Business Acquisition [Line Items]
Acres of developable land | a				758
2015 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Acres of developable land | a				94
2015 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Chicago Minneapolis [Member]
Business Acquisition [Line Items]
Acres of developable land | a				188
2015 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Acres of developable land | a				281
2015 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Acres of developable land | a				56
2015 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Houston [Member]
Business Acquisition [Line Items]
Acres of developable land | a				139
2015 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Business Acquisition [Line Items]
Purchase price | $				$ 220,415
2015 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Purchase price | $				1,727
2015 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Chicago Minneapolis [Member]
Business Acquisition [Line Items]
Purchase price | $				24,541
2015 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Purchase price | $				62,402
2015 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Purchase price | $				45,248
2015 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Houston [Member]
Business Acquisition [Line Items]
Purchase price | $				$ 86,497
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Business Acquisition [Line Items]
Number of buildings | bldg			1
Leaseable square feet | ft²			73,160
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Number of buildings | bldg			0
Leaseable square feet | ft²			0
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Chicago Minneapolis [Member]
Business Acquisition [Line Items]
Number of buildings | bldg			1
Leaseable square feet | ft²			73,160
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Number of buildings | bldg			0
Leaseable square feet | ft²			0
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Philadelphia [Member]
Business Acquisition [Line Items]
Number of buildings | bldg			0
Leaseable square feet | ft²			0
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Southeastern PA [Member]
Business Acquisition [Line Items]
Number of buildings | bldg			0
Leaseable square feet | ft²			0
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Number of buildings | bldg			0
Leaseable square feet | ft²			0
2016 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Business Acquisition [Line Items]
Acres of developable land | a			217
2016 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Acres of developable land | a			13
2016 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Acres of developable land | a			76
2016 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Acres of developable land | a			108
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Business Acquisition [Line Items]
Purchase price | $			$ 60,892
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Purchase price | $			2,251
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Chicago Minneapolis [Member]
Business Acquisition [Line Items]
Purchase price | $			11,064
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Purchase price | $			15,395
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Philadelphia [Member]
Business Acquisition [Line Items]
Purchase price | $			13,943
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Southeastern PA [Member]
Business Acquisition [Line Items]
Purchase price | $			3,344
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Purchase price | $			$ 14,895
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		8
Leaseable square feet | ft²		1,861,879
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		0
Leaseable square feet | ft²		0
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		0
Leaseable square feet | ft²		0
2016 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		8
Leaseable square feet | ft²		1,861,879
2016 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Business Acquisition [Line Items]
Acres of developable land | a		254
2016 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Acres of developable land | a		11
2016 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Acres of developable land | a		95
2016 acquisitions [Member] | Operating Segments [Member] | Land and Land Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Acres of developable land | a		113
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Business Acquisition [Line Items]
Purchase price | $		$ 296,398
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Purchase price | $		1,242
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Purchase price | $		17,798
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | United Kingdom [Member]
Business Acquisition [Line Items]
Purchase price | $		12,550
2016 acquisitions [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Purchase price | $		$ 264,808
Subsequent Event [Member] | 2015 acquisitions [Member] | Operating Segments [Member] | Building and Building Improvements [Member] | Southern California [Member]
Business Acquisition [Line Items]
Leaseable square feet | ft²					400,000
Purchase price | $	$ 92,700