Accounting for the Impairment or Disposal of Long-Lived Assets (Details) (USD $)	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended				3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended							6 Months Ended		3 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Discontinued Operations [Member]	Jun. 30, 2013 Discontinued Operations [Member]	Jun. 30, 2014 Discontinued Operations [Member]	Jun. 30, 2013 Discontinued Operations [Member]	Dec. 31, 2013 Portfolio Sale, Total [Member]	Nov. 30, 2013 Portfolio Sale, Total [Member] properties sqft	Jun. 30, 2014 Portfolio Sale, Total [Member] Discontinued Operations [Member]	Jun. 30, 2013 Portfolio Sale, Total [Member] Discontinued Operations [Member]	Jun. 30, 2014 Portfolio Sale, Total [Member] Discontinued Operations [Member]	Jun. 30, 2013 Portfolio Sale, Total [Member] Discontinued Operations [Member]	Jun. 30, 2014 Other Properties [Member] Discontinued Operations [Member]	Jun. 30, 2013 Other Properties [Member] Discontinued Operations [Member]	Jun. 30, 2014 Other Properties [Member] Discontinued Operations [Member]	Jun. 30, 2013 Other Properties [Member] Discontinued Operations [Member]	Dec. 31, 2013 Portfolio Sale, first settlement [Member] properties acre sqft	Jan. 31, 2014 Portfolio Sale, Second Settlement [Member]	Jan. 31, 2014 Portfolio Sale, Second Settlement [Member] Other Segments [Member] Maryland [Member] properties acre sqft	Jan. 31, 2014 Portfolio Sale, Second Settlement [Member] Other Segments [Member] New Jersey [Member] properties sqft	Jan. 31, 2014 Portfolio Sale, Second Settlement [Member] Southeastern PA [Member] properties sqft	Jun. 30, 2014 Land, Buildings and Improvements [Member] bldg	Jun. 30, 2014 Land, Buildings and Improvements [Member] Other Segments [Member] Not Discontinued Operations [Member] bldg	Jun. 30, 2014 Equity Method Investee [Member] Land, Buildings and Improvements [Member] Other Segments [Member] Not Discontinued Operations [Member] bldg
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties										97									49		23	24	1
Area of Real Estate Property										6,600,000									4,000,000		1,400,000	1,200,000	37,000
Significant Disposals, Sale Proceeds										$ 697,300,000									$ 367,700,000	$ 329,600,000
Number of Real Estate Settlements									2
Area of Land																			140		19
Number of Real Estate Properties Sold																								52	4	3
Proceeds from Sale of Property															0	51,400,000	0	126,000,000						366,900,000	37,300,000	32,200,000
Discontinued Operation, Income (Loss) from Discontinued Operation Disclosures [Abstract]
Revenues											(28,000)	26,659,000	4,733,000	53,800,000
Operating expenses											129,000	(9,113,000)	(2,441,000)	(18,549,000)
Interest and other income											0	57,000	29,000	106,000
Interest expense											0	(3,720,000)	(557,000)	(7,472,000)
Depreciation and Amortization											0	(7,293,000)	0	(14,604,000)
Income before gain on property dispositions											101,000	6,590,000	1,764,000	13,281,000
Gain on property dispositions	140,000	7,658,000	46,256,000	49,364,000							80,000	0	46,121,000	0
Income from discontinued operations	241,000	13,666,000	48,015,000	63,489,000	241,000	13,666,000	48,015,000	63,489,000			181,000	6,590,000	47,885,000	13,281,000	60,000	7,076,000	130,000	50,208,000
Noncontrolling Interest											(4,000)	(195,000)	(1,144,000)	(396,000)
Income from discontinued operations available to common shareholders											177,000	6,395,000	46,741,000	12,885,000
Discontinued Operation, Additional Disclosures [Abstract]
Disposal Group, Including Discontinued Operation, Operating Cash Flows											(97,000)	13,300,000	(2,800,000)	28,000,000
Disposal Group, Including Discontinued Operations, Investing Cash Flows											$ 50,000	$ (4,000,000)	$ 313,200,000	$ (7,800,000)