Acquisitions and Dispositions (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($) ft² a bldg	Sep. 30, 2018 USD ($) ft² a bldg
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	5	5
Leaseable square feet | ft²	805,746	805,746
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 | $	$ 264,700
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	65.8	65.8
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	30	30
Leaseable square feet | ft²	2,514,040	2,514,040
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 589,166
Disposal Group, Not Discontinued Operations [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	78.7	78.7
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	0	0
Leaseable square feet | ft²	0	0
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 | $	$ 0
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	0	0
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Leaseable square feet | ft²	80,000	80,000
Disposal Group, Not Discontinued Operations [Member] | Carolinas Richmond [Member] | Operating Segments [Member] | 2018 sales [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 [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	1.5	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	0
Leaseable square feet | ft²	0	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 | $	$ 0
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	0	0
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
Disposal Group, Not Discontinued Operations [Member] | Chicago Minneapolis [Member] | Operating Segments [Member] | 2018 sales [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 [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	8.3	8.3
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	0
Disposal Group, Not Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central 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	0	0
Leaseable square feet | ft²	0	0
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $	$ 9,700
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central 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	65.8	65.8
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 9,700
Disposal Group, Not Discontinued Operations [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	65.8	65.8
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [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	0
Leaseable square feet | ft²	0	0
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $	$ 0
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [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	0
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Leaseable square feet | ft²	207,779	207,779
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 130,850
Disposal Group, Not Discontinued Operations [Member] | Philadelphia [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	3.1	3.1
Disposal Group, Not Discontinued Operations [Member] | Arizona [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	0
Disposal Group, Not Discontinued Operations [Member] | Arizona [Member] | Operating Segments [Member] | 2018 sales [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
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	0	0
Leaseable square feet | ft²	0	0
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 | $	$ 0
Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	23	23
Leaseable square feet | ft²	1,420,515	1,420,515
Discontinued Operations [Member] | Southeastern PA [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 183,808
Discontinued Operations [Member] | Arizona [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	5	5
Leaseable square feet | ft²	805,746	805,746
Discontinued Operations [Member] | Arizona [Member] | Operating Segments [Member] | 2018 sales current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $	$ 255,000
Discontinued Operations [Member] | Arizona [Member] | Operating Segments [Member] | 2018 sales [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	5	5
Leaseable square feet | ft²	805,746	805,746
Discontinued Operations [Member] | Arizona [Member] | Operating Segments [Member] | 2018 sales [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross proceeds | $		$ 255,000
2018 acquisitions current quarter [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	3	3
Leaseable square feet | ft²	1,312,596	1,312,596
2018 acquisitions current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 150,700
2018 acquisitions current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	38.4	38.4
2018 acquisitions current quarter [Member] | Dallas [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Leaseable square feet | ft²	900,043	900,043
2018 acquisitions current quarter [Member] | Dallas [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 52,491
2018 acquisitions current quarter [Member] | Dallas [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
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	0
Leaseable square feet | ft²	0	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 | $	$ 709
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	1.6	1.6
2018 acquisitions current quarter [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions current quarter [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 3,000
2018 acquisitions current quarter [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	16.4	16.4
2018 acquisitions current quarter [Member] | Florida [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions current quarter [Member] | Florida [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 0
2018 acquisitions current quarter [Member] | Florida [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
2018 acquisitions current quarter [Member] | New Jersey [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	2	2
Leaseable square feet | ft²	412,553	412,553
2018 acquisitions current quarter [Member] | New Jersey [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 61,375
2018 acquisitions current quarter [Member] | New Jersey [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	7.5	7.5
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	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions current quarter [Member] | Southern California [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 33,125
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	12.9	12.9
2018 acquisitions [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	7	7
Leaseable square feet | ft²	2,487,083	2,487,083
2018 acquisitions [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	250.8	250.8
2018 acquisitions [Member] | Dallas [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	1	1
Leaseable square feet | ft²	900,043	900,043
2018 acquisitions [Member] | Dallas [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 52,491
2018 acquisitions [Member] | Dallas [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	0	0
2018 acquisitions [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	0
Leaseable square feet | ft²	0	0
2018 acquisitions [Member] | United Kingdom [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 4,949
2018 acquisitions [Member] | United Kingdom [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	8.7	8.7
2018 acquisitions [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 34,043
2018 acquisitions [Member] | Lehigh Central PA [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	170.2	170.2
2018 acquisitions [Member] | Florida [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	0	0
Leaseable square feet | ft²	0	0
2018 acquisitions [Member] | Florida [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 14,728
2018 acquisitions [Member] | Florida [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	28.4	28.4
2018 acquisitions [Member] | New Jersey [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	3	3
Leaseable square feet | ft²	584,569	584,569
2018 acquisitions [Member] | New Jersey [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 84,875
2018 acquisitions [Member] | New Jersey [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	7.5	7.5
2018 acquisitions [Member] | Southern California [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 422,147
2018 acquisitions [Member] | Southern California [Member] | Operating Segments [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	3	3
Leaseable square feet | ft²	1,002,471	1,002,471
2018 acquisitions [Member] | Southern California [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 231,061
2018 acquisitions [Member] | Southern California [Member] | Operating Segments [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	36.0	36.0