Real Estate (Real Estate Properties Sold) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft² a bldg	Dec. 31, 2016 USD ($) ft² a bldg	Dec. 31, 2015 USD ($) ft² a bldg
Disposition Disclosures [Line Items]
Number of buildings | bldg	37
Acres of developable land	9
Gross proceeds | $	$ 379,139	$ 1,192,918	$ 530,440
Building and Building Improvements [Member] | United Kingdom [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	0
Leaseable square feet | ft²	0
Land and Land Improvements [Member] | Philadelphia [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land		6
Land and Land Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land		8
Land and Land Improvements [Member] | United Kingdom [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land	35
Land and Land Improvements [Member] | Chicago Minneapolis [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land		6
2016 sales [Member] | Richmond Hampton Roads Sale [Member] | Immediate Family Member of Management or Principal Owner [Member] | Building and Building Improvements [Member] | Carolinas Richmond [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg			22
2016 sales [Member] | Richmond Hampton Roads Sale [Member] | Immediate Family Member of Management or Principal Owner [Member] | Land and Land Improvements [Member] | Carolinas Richmond [Member]
Disposition Disclosures [Line Items]
Acres of developable land			3
2016 sales [Member] | Richmond Hampton Roads Sale [Member] | Immediate Family Member of Management or Principal Owner [Member] | Land, Buildings and Improvements [Member] | Carolinas Richmond [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $			$ 110,300
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		125
Leaseable square feet | ft²		9,463,990
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		51
Leaseable square feet | ft²		3,509,641
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Lehigh Central PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		1
Leaseable square feet | ft²		120,777
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Philadelphia [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		0
Leaseable square feet | ft²		0
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		18
Leaseable square feet | ft²		1,631,295
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		31
Leaseable square feet | ft²		2,165,002
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Chicago Minneapolis [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		24
Leaseable square feet | ft²		2,037,275
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land		36
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land		12
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Lehigh Central PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land		0
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Philadelphia [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land		1
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land		18
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land		0
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Chicago Minneapolis [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land		5
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $		$ 1,210,555
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $		505,828
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Lehigh Central PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $		11,200
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Philadelphia [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $		2,640
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $		224,241
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $		266,821
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Chicago Minneapolis [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $		199,825
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg			81
Leaseable square feet | ft²			5,710,173
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg			4
Leaseable square feet | ft²			874,125
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg			7
Leaseable square feet | ft²			569,618
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Southeastern PA [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg			41
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg			46
Leaseable square feet | ft²			2,724,489
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Carolinas Richmond [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg			22
Leaseable square feet | ft²			1,319,299
Not Discontinued Operations [Member] | 2016 sales [Member] | Building and Building Improvements [Member] | Chicago Minneapolis [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg			2
Leaseable square feet | ft²			222,642
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land			23
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land			0
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land			0
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Southeastern PA [Member]
Disposition Disclosures [Line Items]
Acres of developable land			20
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land			20
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Carolinas Richmond [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land			3
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Chicago Minneapolis [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land			0
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $			$ 545,133
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $			55,425
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $			37,371
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Southeastern PA [Member]
Disposition Disclosures [Line Items]
Gain on sale of property | $		$ 14,300
Gain (Loss) on Disposition of Property Plant Equipment | $	$ 14,300
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $			316,583
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Carolinas Richmond [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $			110,054
Not Discontinued Operations [Member] | 2016 sales [Member] | Land, Buildings and Improvements [Member] | Chicago Minneapolis [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $			$ 25,700
Not Discontinued Operations [Member] | 2017 sales [Member] | Building and Building Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	10
Leaseable square feet | ft²	2,340,204
Not Discontinued Operations [Member] | 2017 sales [Member] | Building and Building Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	0
Leaseable square feet | ft²	0
Not Discontinued Operations [Member] | 2017 sales [Member] | Building and Building Improvements [Member] | Houston [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	1
Leaseable square feet | ft²	206,808
Not Discontinued Operations [Member] | 2017 sales [Member] | Building and Building Improvements [Member] | Lehigh Central PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	2
Leaseable square feet | ft²	1,683,876
Not Discontinued Operations [Member] | 2017 sales [Member] | Building and Building Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	0
Leaseable square feet | ft²	0
Not Discontinued Operations [Member] | 2017 sales [Member] | Building and Building Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	4
Leaseable square feet | ft²	313,410
Not Discontinued Operations [Member] | 2017 sales [Member] | Building and Building Improvements [Member] | Chicago Minneapolis [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	3
Leaseable square feet | ft²	136,110
Not Discontinued Operations [Member] | 2017 sales [Member] | Land and Land Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land	113
Not Discontinued Operations [Member] | 2017 sales [Member] | Land and Land Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land	16
Not Discontinued Operations [Member] | 2017 sales [Member] | Land and Land Improvements [Member] | Houston [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land	42
Not Discontinued Operations [Member] | 2017 sales [Member] | Land and Land Improvements [Member] | Lehigh Central PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land	44
Not Discontinued Operations [Member] | 2017 sales [Member] | Land and Land Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land	8
Not Discontinued Operations [Member] | 2017 sales [Member] | Land and Land Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land	3
Not Discontinued Operations [Member] | 2017 sales [Member] | Land and Land Improvements [Member] | Chicago Minneapolis [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land	0
Not Discontinued Operations [Member] | 2017 sales [Member] | Land, Buildings and Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $	$ 367,258
Not Discontinued Operations [Member] | 2017 sales [Member] | Land, Buildings and Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $	13,256
Not Discontinued Operations [Member] | 2017 sales [Member] | Land, Buildings and Improvements [Member] | Houston [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $	36,145
Not Discontinued Operations [Member] | 2017 sales [Member] | Land, Buildings and Improvements [Member] | Lehigh Central PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $	249,045
Not Discontinued Operations [Member] | 2017 sales [Member] | Land, Buildings and Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $	3,739
Not Discontinued Operations [Member] | 2017 sales [Member] | Land, Buildings and Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $	56,156
Not Discontinued Operations [Member] | 2017 sales [Member] | Land, Buildings and Improvements [Member] | Chicago Minneapolis [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds | $	$ 8,917