Building Acquisitions and Dispositions (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($) ft² a bldg	Jun. 30, 2017 USD ($) ft² a bldg	Jun. 30, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Buildings and Land		$ 26,220	$ 139,814
Disposal Group, Not Discontinued Operations [Member] | 2017 sales [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	3	3
Acres of developable land | a	29	29
Leaseable square feet | ft²	126,529	126,529
Proceeds from Sale of Buildings and Land		$ 27,424
Disposal Group, Not Discontinued Operations [Member] | 2017 sales [Member] | Operating Segments [Member] | Chicago Minneapolis [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Acres of developable land | a	0	0
Leaseable square feet | ft²	31,116	31,116
Proceeds from Sale of Buildings and Land		$ 3,100
Disposal Group, Not Discontinued Operations [Member] | 2017 sales [Member] | Operating Segments [Member] | Florida [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Acres of developable land | a	11	11
Leaseable square feet | ft²	0	0
Proceeds from Sale of Buildings and Land		$ 2,636
Disposal Group, Not Discontinued Operations [Member] | 2017 sales [Member] | Operating Segments [Member] | Houston [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Acres of developable land | a	7	7
Leaseable square feet | ft²	0	0
Proceeds from Sale of Buildings and Land		$ 1,995
Disposal Group, Not Discontinued Operations [Member] | 2017 sales [Member] | Operating Segments [Member] | Philadelphia [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Acres of developable land | a	2	2
Leaseable square feet | ft²	0	0
Proceeds from Sale of Buildings and Land		$ 6,904
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]
Number of Real Estate Properties | bldg	2	2
Acres of developable land | a	3	3
Leaseable square feet | ft²	95,413	95,413
Proceeds from Sale of Buildings and Land		$ 9,050
Disposal Group, Not Discontinued Operations [Member] | 2017 sales [Member] | Operating Segments [Member] | Other Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Acres of developable land | a	6	6
Leaseable square feet | ft²	0	0
Proceeds from Sale of Buildings and Land		$ 3,739
Disposal Group, Not Discontinued Operations [Member] | 2017 sales current quarter [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	2	2
Acres of developable land | a	29	29
Leaseable square feet | ft²	93,502	93,502
Proceeds from Sale of Buildings and Land	$ 25,349
Disposal Group, Not Discontinued Operations [Member] | 2017 sales current quarter [Member] | Operating Segments [Member] | Chicago Minneapolis [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Acres of developable land | a	0	0
Leaseable square feet | ft²	31,116	31,116
Proceeds from Sale of Buildings and Land	$ 3,100
Disposal Group, Not Discontinued Operations [Member] | 2017 sales current quarter [Member] | Operating Segments [Member] | Florida [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Acres of developable land | a	11	11
Leaseable square feet | ft²	0	0
Proceeds from Sale of Buildings and Land	$ 2,636
Disposal Group, Not Discontinued Operations [Member] | 2017 sales current quarter [Member] | Operating Segments [Member] | Houston [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Acres of developable land | a	7	7
Leaseable square feet | ft²	0	0
Proceeds from Sale of Buildings and Land	$ 1,995
Disposal Group, Not Discontinued Operations [Member] | 2017 sales current quarter [Member] | Operating Segments [Member] | Philadelphia [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Acres of developable land | a	2	2
Leaseable square feet | ft²	0	0
Proceeds from Sale of Buildings and Land	$ 6,904
Disposal Group, Not Discontinued Operations [Member] | 2017 sales current quarter [Member] | Operating Segments [Member] | Southeastern PA [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Acres of developable land | a	3	3
Leaseable square feet | ft²	62,386	62,386
Proceeds from Sale of Buildings and Land	$ 6,975
Disposal Group, Not Discontinued Operations [Member] | 2017 sales current quarter [Member] | Operating Segments [Member] | Other Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Acres of developable land | a	6	6
Leaseable square feet | ft²	0	0
Proceeds from Sale of Buildings and Land	$ 3,739
2017 acquisitions current quarter [Member] | Operating Segments [Member] | Southern California [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Leaseable square feet | ft²	101,000	101,000
Purchase price	$ 19,100
2017 acquisitions [Member] | Operating Segments [Member] | Southern California [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Leaseable square feet | ft²	101,000	101,000
Purchase price		$ 19,100