Acquisitions and Dispositions (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($) ft² a bldg	Jun. 30, 2018 USD ($) 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	1	1
Leaseable square feet | ft²	207,779	207,779
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 | $	$ 130,850
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	3.1	3.1
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	25	25
Leaseable square feet | ft²	1,708,294	1,708,294
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 | $		$ 324,466
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	12.9	12.9
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] | 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	1	1
Leaseable square feet | ft²	207,779	207,779
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 | $	$ 130,850
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	3.1	3.1
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
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
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	774,318	774,318
2018 acquisitions current quarter [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 174,507
2018 acquisitions current quarter [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	204.5	204.5
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 | $	$ 0
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	0	0
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 | $	$ 31,043
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	153.8	153.8
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 | $	$ 14,728
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	28.4	28.4
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	1	1
Leaseable square feet | ft²	172,016	172,016
2018 acquisitions current quarter [Member] | New Jersey [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 23,500
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	0	0
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	2	2
Leaseable square feet | ft²	602,302	602,302
2018 acquisitions current quarter [Member] | Southern California [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $	$ 105,236
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	22.3	22.3
2018 acquisitions [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties | bldg	4	4
Leaseable square feet | ft²	1,174,487	1,174,487
2018 acquisitions [Member] | Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Acres of Developable Land | a	212.4	212.4
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 | $		$ 4,240
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	7.1	7.1
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 | $		$ 31,043
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	153.8	153.8
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	1	1
Leaseable square feet | ft²	172,016	172,016
2018 acquisitions [Member] | New Jersey [Member] | Operating Segments [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 23,500
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	0	0
2018 acquisitions [Member] | Southern California [Member] | Land, Buildings and Improvements [Member]
Property, Plant and Equipment [Line Items]
Purchase price | $		$ 271,447
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 | $		$ 197,936
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	23.1	23.1