Real Estate (Real Estate Properties Sold) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) a ft² bldg	Dec. 31, 2017 USD ($) a ft² bldg	Dec. 31, 2016 USD ($) a ft² bldg	Dec. 31, 2015 a bldg
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg	60
Land and Land Improvements | Florida | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a	30.8
Not Discontinued Operations | 2018 sales | Building and Building Improvements | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg	39
Leaseable square feet (in sqft) | ft²	3,418,564
Not Discontinued Operations | 2018 sales | Building and Building Improvements | Carolinas/Richmond | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg	1
Leaseable square feet (in sqft) | ft²	80,000
Not Discontinued Operations | 2018 sales | Building and Building Improvements | Chicago/Minneapolis | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg	0
Leaseable square feet (in sqft) | ft²	0
Not Discontinued Operations | 2018 sales | Building and Building Improvements | Lehigh/Central PA | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg	0
Leaseable square feet (in sqft) | ft²	0
Not Discontinued Operations | 2018 sales | Building and Building Improvements | Philadelphia | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg	1
Leaseable square feet (in sqft) | ft²	207,779
Not Discontinued Operations | 2018 sales | Building and Building Improvements | Southeastern PA | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg	32
Leaseable square feet (in sqft) | ft²	2,325,039
Not Discontinued Operations | 2018 sales | Building and Building Improvements | United Kingdom | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg	0
Leaseable square feet (in sqft) | ft²	0
Not Discontinued Operations | 2018 sales | Building and Building Improvements | Other | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg	5
Leaseable square feet (in sqft) | ft²	805,746
Not Discontinued Operations | 2018 sales | Land and Land Improvements | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a	79.1
Not Discontinued Operations | 2018 sales | Land and Land Improvements | Carolinas/Richmond | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a	1.5
Not Discontinued Operations | 2018 sales | Land and Land Improvements | Chicago/Minneapolis | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a	8.3
Not Discontinued Operations | 2018 sales | Land and Land Improvements | Lehigh/Central PA | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a	66.2
Not Discontinued Operations | 2018 sales | Land and Land Improvements | Philadelphia | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a	3.1
Not Discontinued Operations | 2018 sales | Land and Land Improvements | Southeastern PA | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a	0
Not Discontinued Operations | 2018 sales | Land and Land Improvements | United Kingdom | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a	0
Not Discontinued Operations | 2018 sales | Land and Land Improvements | Other | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a	0
Not Discontinued Operations | 2018 sales | Land, Buildings and Improvements | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $	$ 873,368
Not Discontinued Operations | 2018 sales | Land, Buildings and Improvements | Carolinas/Richmond | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $	7,094
Not Discontinued Operations | 2018 sales | Land, Buildings and Improvements | Chicago/Minneapolis | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $	2,714
Not Discontinued Operations | 2018 sales | Land, Buildings and Improvements | Lehigh/Central PA | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $	9,700
Not Discontinued Operations | 2018 sales | Land, Buildings and Improvements | Philadelphia | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $	130,818
Not Discontinued Operations | 2018 sales | Land, Buildings and Improvements | Southeastern PA | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $	402,708
Not Discontinued Operations | 2018 sales | Land, Buildings and Improvements | United Kingdom | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $	65,334
Not Discontinued Operations | 2018 sales | Land, Buildings and Improvements | Other | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $	255,000
Not Discontinued Operations | 2017 sales | Building and Building Improvements | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg		10
Leaseable square feet (in sqft) | ft²		2,340,204
Not Discontinued Operations | 2017 sales | Building and Building Improvements | Chicago/Minneapolis | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg		3
Leaseable square feet (in sqft) | ft²		136,110
Not Discontinued Operations | 2017 sales | Building and Building Improvements | Florida | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg		0
Leaseable square feet (in sqft) | ft²		0
Not Discontinued Operations | 2017 sales | Building and Building Improvements | Houston | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg		1
Leaseable square feet (in sqft) | ft²		206,808
Not Discontinued Operations | 2017 sales | Building and Building Improvements | Lehigh/Central PA | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg		2
Leaseable square feet (in sqft) | ft²		1,683,876
Not Discontinued Operations | 2017 sales | Building and Building Improvements | Southeastern PA | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg		4
Leaseable square feet (in sqft) | ft²		313,410
Not Discontinued Operations | 2017 sales | Building and Building Improvements | Other | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg		0
Leaseable square feet (in sqft) | ft²		0
Not Discontinued Operations | 2017 sales | Land and Land Improvements | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a		113.1
Not Discontinued Operations | 2017 sales | Land and Land Improvements | Chicago/Minneapolis | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a		0.0
Not Discontinued Operations | 2017 sales | Land and Land Improvements | Florida | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a		16.4
Not Discontinued Operations | 2017 sales | Land and Land Improvements | Houston | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a		42.2
Not Discontinued Operations | 2017 sales | Land and Land Improvements | Lehigh/Central PA | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a		44.3
Not Discontinued Operations | 2017 sales | Land and Land Improvements | Southeastern PA | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a		3.0
Not Discontinued Operations | 2017 sales | Land and Land Improvements | Other | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a		7.2
Not Discontinued Operations | 2017 sales | Land, Buildings and Improvements | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $		$ 367,258
Not Discontinued Operations | 2017 sales | Land, Buildings and Improvements | Chicago/Minneapolis | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $		8,917
Not Discontinued Operations | 2017 sales | Land, Buildings and Improvements | Florida | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $		13,256
Not Discontinued Operations | 2017 sales | Land, Buildings and Improvements | Houston | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $		36,145
Not Discontinued Operations | 2017 sales | Land, Buildings and Improvements | Lehigh/Central PA | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $		249,045
Not Discontinued Operations | 2017 sales | Land, Buildings and Improvements | Southeastern PA | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $		56,156
Not Discontinued Operations | 2017 sales | Land, Buildings and Improvements | Other | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $		$ 3,739
Not Discontinued Operations | 2016 sales | Building and Building Improvements | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg			125
Leaseable square feet (in sqft) | ft²			9,463,990
Not Discontinued Operations | 2016 sales | Building and Building Improvements | Chicago/Minneapolis | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg			24
Leaseable square feet (in sqft) | ft²			2,037,275
Not Discontinued Operations | 2016 sales | Building and Building Improvements | Florida | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg			51
Leaseable square feet (in sqft) | ft²			3,509,641
Not Discontinued Operations | 2016 sales | Building and Building Improvements | Lehigh/Central PA | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg			1
Leaseable square feet (in sqft) | ft²			120,777
Not Discontinued Operations | 2016 sales | Building and Building Improvements | Philadelphia | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg			0
Leaseable square feet (in sqft) | ft²			0
Not Discontinued Operations | 2016 sales | Building and Building Improvements | Southeastern PA | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg			31
Leaseable square feet (in sqft) | ft²			2,165,002
Not Discontinued Operations | 2016 sales | Building and Building Improvements | Other | Operating Segments
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg			18
Leaseable square feet (in sqft) | ft²			1,631,295
Not Discontinued Operations | 2016 sales | Land and Land Improvements | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a			36.4
Not Discontinued Operations | 2016 sales | Land and Land Improvements | Chicago/Minneapolis | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a			5.3
Not Discontinued Operations | 2016 sales | Land and Land Improvements | Florida | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a			12.1
Not Discontinued Operations | 2016 sales | Land and Land Improvements | Lehigh/Central PA | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a			0.0
Not Discontinued Operations | 2016 sales | Land and Land Improvements | Philadelphia | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a			1.0
Not Discontinued Operations | 2016 sales | Land and Land Improvements | Southeastern PA | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a			0.0
Not Discontinued Operations | 2016 sales | Land and Land Improvements | Other | Operating Segments
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a			18.0
Not Discontinued Operations | 2016 sales | Land, Buildings and Improvements | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $			$ 1,210,555
Not Discontinued Operations | 2016 sales | Land, Buildings and Improvements | Chicago/Minneapolis | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $			199,825
Not Discontinued Operations | 2016 sales | Land, Buildings and Improvements | Florida | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $			505,828
Not Discontinued Operations | 2016 sales | Land, Buildings and Improvements | Lehigh/Central PA | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $			11,200
Not Discontinued Operations | 2016 sales | Land, Buildings and Improvements | Philadelphia | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $			2,640
Not Discontinued Operations | 2016 sales | Land, Buildings and Improvements | Southeastern PA | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $			266,821
Not Discontinued Operations | 2016 sales | Land, Buildings and Improvements | Other | Operating Segments
Disposition Disclosures [Line Items]
Gross proceeds | $			224,241
Not Discontinued Operations | 2015 sales | Building and Building Improvements | Southeastern PA
Disposition Disclosures [Line Items]
Number of buildings (in buildings) | bldg				41
Not Discontinued Operations | 2015 sales | Land and Land Improvements | Southeastern PA
Disposition Disclosures [Line Items]
Acres of developable land (in acres) | a				20
Not Discontinued Operations | 2015 sales | Land, Buildings and Improvements | Southeastern PA
Disposition Disclosures [Line Items]
Gain on sale of property | $			$ 14,300
Recognized deferred gain | $	$ 14,300