Real Estate (Real Estate Properties Acquired) (Details) - Operating Segments [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) a ft² bldg	Dec. 31, 2015 USD ($) a ft² bldg
2016 acquisitions [Member] | Building and Building Improvements [Member]
Business Acquisition [Line Items]
Number of buildings | bldg	1
Leaseable square feet | ft²	73,160
2016 acquisitions [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] | Building and Building Improvements [Member] | Chicago Milwaukee [Member]
Business Acquisition [Line Items]
Number of buildings | bldg	1
Leaseable square feet | ft²	73,160
2016 acquisitions [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] | Building and Building Improvements [Member] | Philadelphia [Member]
Business Acquisition [Line Items]
Number of buildings | bldg	0
Leaseable square feet | ft²	0
2016 acquisitions [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] | 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] | Land and Land Improvements [Member]
Business Acquisition [Line Items]
Acres of developable land | a	217
2016 acquisitions [Member] | Land and Land Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Acres of developable land | a	13
2016 acquisitions [Member] | Land and Land Improvements [Member] | Chicago Milwaukee [Member]
Business Acquisition [Line Items]
Acres of developable land | a	6
2016 acquisitions [Member] | Land and Land Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Acres of developable land | a	76
2016 acquisitions [Member] | Land and Land Improvements [Member] | Philadelphia [Member]
Business Acquisition [Line Items]
Acres of developable land | a	6
2016 acquisitions [Member] | Land and Land Improvements [Member] | Southeastern PA [Member]
Business Acquisition [Line Items]
Acres of developable land | a	8
2016 acquisitions [Member] | Land and Land Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Acres of developable land | a	108
2016 acquisitions [Member] | Land, Buildings and Improvements [Member]
Business Acquisition [Line Items]
Purchase price | $	$ 60,892
2016 acquisitions [Member] | Land, Buildings and Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Purchase price | $	2,251
2016 acquisitions [Member] | Land, Buildings and Improvements [Member] | Chicago Milwaukee [Member]
Business Acquisition [Line Items]
Purchase price | $	11,064
2016 acquisitions [Member] | Land, Buildings and Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Purchase price | $	15,395
2016 acquisitions [Member] | Land, Buildings and Improvements [Member] | Philadelphia [Member]
Business Acquisition [Line Items]
Purchase price | $	13,943
2016 acquisitions [Member] | Land, Buildings and Improvements [Member] | Southeastern PA [Member]
Business Acquisition [Line Items]
Purchase price | $	3,344
2016 acquisitions [Member] | Land, Buildings and Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Purchase price | $	$ 14,895
2015 acquisitions [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] | 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] | 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] | 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] | 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] | Building and Building Improvements [Member] | Minnesota [Member]
Business Acquisition [Line Items]
Number of buildings | bldg		1
Leaseable square feet | ft²		197,956
2015 acquisitions [Member] | Land and Land Improvements [Member]
Business Acquisition [Line Items]
Acres of developable land | a		758
2015 acquisitions [Member] | Land and Land Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Acres of developable land | a		94
2015 acquisitions [Member] | Land and Land Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Acres of developable land | a		281
2015 acquisitions [Member] | Land and Land Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Acres of developable land | a		56
2015 acquisitions [Member] | Land and Land Improvements [Member] | Houston [Member]
Business Acquisition [Line Items]
Acres of developable land | a		139
2015 acquisitions [Member] | Land and Land Improvements [Member] | Minnesota [Member]
Business Acquisition [Line Items]
Acres of developable land | a		188
2015 acquisitions [Member] | Land, Buildings and Improvements [Member]
Business Acquisition [Line Items]
Purchase price | $		$ 220,415
2015 acquisitions [Member] | Land, Buildings and Improvements [Member] | Carolinas Richmond [Member]
Business Acquisition [Line Items]
Purchase price | $		1,727
2015 acquisitions [Member] | Land, Buildings and Improvements [Member] | Lehigh Central PA [Member]
Business Acquisition [Line Items]
Purchase price | $		62,402
2015 acquisitions [Member] | Land, Buildings and Improvements [Member] | Other Segments [Member]
Business Acquisition [Line Items]
Purchase price | $		45,248
2015 acquisitions [Member] | Land, Buildings and Improvements [Member] | Houston [Member]
Business Acquisition [Line Items]
Purchase price | $		86,497
2015 acquisitions [Member] | Land, Buildings and Improvements [Member] | Minnesota [Member]
Business Acquisition [Line Items]
Purchase price | $		$ 24,541