Real Estate (Real Estate Properties Sold) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) a ft² bldg	Dec. 31, 2015 USD ($) a ft² bldg	Dec. 31, 2014 USD ($)
Disposition Disclosures [Line Items]
Gross proceeds	$ 1,192,918	$ 530,440	$ 559,322
2015 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
2015 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 | a		3
2015 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] | Chicago Milwaukee [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	1
Leaseable square feet | ft²	171,114
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] | Minnesota [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	23
Leaseable square feet | ft²	1,866,161
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] | Land and Land Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land | a	36
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Chicago Milwaukee [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land | a	5
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 | a	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 | a	0
Not Discontinued Operations [Member] | 2016 sales [Member] | Land and Land Improvements [Member] | Minnesota [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land | a	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 | a	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 | a	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 | a	0
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] | Chicago Milwaukee [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds	10,625
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] | Minnesota [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds	189,200
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] | 2015 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] | 2015 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] | 2015 sales [Member] | Building and Building Improvements [Member] | Minnesota [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		2
Leaseable square feet | ft²		222,642
Not Discontinued Operations [Member] | 2015 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] | 2015 sales [Member] | Building and Building Improvements [Member] | Southeastern PA [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		41
Not Discontinued Operations [Member] | 2015 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] | 2015 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] | 2015 sales [Member] | Land and Land Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land | a		23
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land | a		0
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Minnesota [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land | a		0
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land | a		0
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Southeastern PA [Member]
Disposition Disclosures [Line Items]
Acres of developable land | a		20
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land | a		20
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Carolinas Richmond [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land | a		3
Not Discontinued Operations [Member] | 2015 sales [Member] | Land, Buildings and Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds		$ 545,133
Not Discontinued Operations [Member] | 2015 sales [Member] | Land, Buildings and Improvements [Member] | Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds		55,425
Not Discontinued Operations [Member] | 2015 sales [Member] | Land, Buildings and Improvements [Member] | Minnesota [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds		25,700
Not Discontinued Operations [Member] | 2015 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] | 2015 sales [Member] | Land, Buildings and Improvements [Member] | Southeastern PA [Member]
Disposition Disclosures [Line Items]
Gain on sale of property	$ 14,300	14,300
Not Discontinued Operations [Member] | 2015 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] | 2015 sales [Member] | Land, Buildings and Improvements [Member] | Carolinas Richmond [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Gross proceeds		$ 110,054