Real Estate Properties Sold (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) a ft² bldg	Dec. 31, 2014 USD ($) a ft² bldg	Dec. 31, 2013 USD ($)	Jan. 01, 2014 a ft² bldg
Disposition Disclosures [Line Items]
Gross Proceeds | $	$ 530,440	$ 559,322	$ 526,949
Gain on property dispositions | $	100,314	45,147	0
Discontinued operations, net gain on property dispositions | $	$ 0	$ 46,631	95,384
Land and Land Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	758	502
Land and Land Improvements [Member] | Carolinas [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)		104
Land and Land Improvements [Member] | Chicago Milwaukee [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)		16
Land and Land Improvements [Member] | Houston [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	139	110
Land and Land Improvements [Member] | Lehigh Central PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	281	85
Land and Land Improvements [Member] | Minnesota [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	188	24
Land and Land Improvements [Member] | Richmond Hampton Roads [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	94
Land and Land Improvements [Member] | South Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)		69
Land and Land Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	56	94
2015 sales [Member] | Richmond Hampton Roads Sale [Member] | Immediate Family Member of Management or Principal Owner [Member] | Land and Land Improvements [Member] | Richmond Hampton Roads [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	3
2015 sales [Member] | Richmond Hampton Roads Sale [Member] | Immediate Family Member of Management or Principal Owner [Member] | Land, Buildings and Improvements [Member] | Richmond Hampton Roads [Member]
Disposition Disclosures [Line Items]
Gross Proceeds | $	$ 110,300
2015 sales [Member] | Richmond Hampton Roads Sale [Member] | Immediate Family Member of Management or Principal Owner [Member] | Building and Building Improvements [Member] | Richmond Hampton Roads [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	22
2014 Sales [Member] | Land and Land Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)		70
2014 Sales [Member] | Building and Building Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		65
Leaseable square feet (unaudited) | ft²		4,740,329
Gross Proceeds | $		$ 558,578
Discontinued operations and disposal sale, not discontinued operations [Member] | 2014 Sales [Member] | New Jersey [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		27
Leaseable square feet (unaudited) | ft²		1,790,893
Gross Proceeds | $		$ 164,767
Discontinued operations and disposal sale, not discontinued operations [Member] | 2014 Sales [Member] | Building and Building Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		3
Leaseable square feet (unaudited) | ft²		113,833
Gross Proceeds | $		$ 12,225
Not Discontinued Operations [Member] | Minnesota [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	1
Leaseable square feet (unaudited) | ft²	92,000
Not Discontinued Operations [Member] | Land and Land Improvements [Member] | Maryland [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)				19
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	23
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 (unaudited)	0
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Orlando [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	0
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Richmond Hampton Roads [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	3
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | South Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	0
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Southeastern PA [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	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 (unaudited)	20
Not Discontinued Operations [Member] | 2015 sales [Member] | Land and Land Improvements [Member] | Tampa [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)	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 (unaudited)	0
Not Discontinued Operations [Member] | 2015 sales [Member] | Land, Buildings and Improvements [Member] | Southeastern PA [Member]
Disposition Disclosures [Line Items]
Deferred Gain on Sale of Property | $	$ 14,300
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 (unaudited) | ft²	5,710,173
Gross Proceeds | $	$ 545,133
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 (unaudited) | ft²	222,642
Gross Proceeds | $	$ 25,700
Not Discontinued Operations [Member] | 2015 sales [Member] | Building and Building Improvements [Member] | Orlando [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	1
Leaseable square feet (unaudited) | ft²	713,585
Gross Proceeds | $	$ 35,500
Not Discontinued Operations [Member] | 2015 sales [Member] | Building and Building Improvements [Member] | Richmond Hampton Roads [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	22
Leaseable square feet (unaudited) | ft²	1,319,299
Gross Proceeds | $	$ 110,054
Not Discontinued Operations [Member] | 2015 sales [Member] | Building and Building Improvements [Member] | South Florida [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	1
Leaseable square feet (unaudited) | ft²	99,740
Gross Proceeds | $	$ 14,700
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 (unaudited) | ft²	2,724,489
Gross Proceeds | $	$ 316,583
Not Discontinued Operations [Member] | 2015 sales [Member] | Building and Building Improvements [Member] | Tampa [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg	2
Leaseable square feet (unaudited) | ft²	60,800
Gross Proceeds | $	$ 5,225
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 (unaudited) | ft²	569,618
Gross Proceeds | $	$ 37,371
Not Discontinued Operations [Member] | 2014 Sales [Member] | Houston [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		5
Acres of developable land (unaudited)		0
Leaseable square feet (unaudited) | ft²		553,510
Gross Proceeds | $		$ 29,721
Not Discontinued Operations [Member] | 2014 Sales [Member] | Maryland [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)		19
Not Discontinued Operations [Member] | 2014 Sales [Member] | New Jersey [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)		51
Not Discontinued Operations [Member] | 2014 Sales [Member] | Land and Land Improvements [Member] | Minnesota [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)		0
Not Discontinued Operations [Member] | 2014 Sales [Member] | Land and Land Improvements [Member] | Orlando [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)		0
Not Discontinued Operations [Member] | 2014 Sales [Member] | Land and Land Improvements [Member] | Southeastern PA [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)		0
Not Discontinued Operations [Member] | 2014 Sales [Member] | Land and Land Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Acres of developable land (unaudited)		0
Not Discontinued Operations [Member] | 2014 Sales [Member] | Building and Building Improvements [Member] | Minnesota [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		1
Leaseable square feet (unaudited) | ft²		191,336
Gross Proceeds | $		$ 41,000
Not Discontinued Operations [Member] | 2014 Sales [Member] | Building and Building Improvements [Member] | Orlando [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		4
Leaseable square feet (unaudited) | ft²		588,678
Gross Proceeds | $		$ 116,750
Not Discontinued Operations [Member] | 2014 Sales [Member] | Building and Building Improvements [Member] | Other Segments [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		2
Leaseable square feet (unaudited) | ft²		134,510
Gross Proceeds | $		$ 8,655
Not Discontinued Operations [Member] | Gain (loss) on property dispositions [Member] | 2015 sales [Member]
Disposition Disclosures [Line Items]
Gain on property dispositions | $	100,300
Not Discontinued Operations [Member] | Gain (loss) on property dispositions [Member] | 2014 Sales [Member]
Disposition Disclosures [Line Items]
Gain on property dispositions | $		45,100
Discontinued Operations [Member]
Disposition Disclosures [Line Items]
Discontinued operations, net gain on property dispositions | $	$ 0	$ 46,631	95,384
Discontinued Operations [Member] | Building and Building Improvements [Member] | Maryland [Member]
Disposition Disclosures [Line Items]
Leaseable square feet (unaudited) | ft²				1,400,000
Discontinued Operations [Member] | Building and Building Improvements [Member] | New Jersey [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg				24
Discontinued Operations [Member] | Building and Building Improvements [Member] | Southeastern PA [Member]
Disposition Disclosures [Line Items]
Leaseable square feet (unaudited) | ft²				37,000
Discontinued Operations [Member] | 2014 Sales [Member] | Maryland [Member] | Operating Segments [Member]
Disposition Disclosures [Line Items]
Number of buildings | bldg		23
Leaseable square feet (unaudited) | ft²		1,367,569
Gross Proceeds | $		$ 185,460
Discontinued Operations [Member] | Discontinued Operations [Member] | 2014 Sales [Member]
Disposition Disclosures [Line Items]
Discontinued operations, net gain on property dispositions | $		$ 46,600
Discontinued Operations [Member] | Discontinued Operations [Member] | 2013 sales [Member]
Disposition Disclosures [Line Items]
Discontinued operations, net gain on property dispositions | $			$ 96,300