Accounting for the Impairment or Disposal of Long-Lived Assets (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended					3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended							3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Discontinued Operations [Member]	Sep. 30, 2013 Discontinued Operations [Member]	Sep. 30, 2014 Discontinued Operations [Member]	Sep. 30, 2013 Discontinued Operations [Member]	Sep. 30, 2014 Other Segments [Member] sqft	Dec. 31, 2013 Portfolio Sale, Total [Member]	Nov. 30, 2013 Portfolio Sale, Total [Member] sqft properties	Sep. 30, 2014 Portfolio Sale, Total [Member] Discontinued Operations [Member]	Sep. 30, 2013 Portfolio Sale, Total [Member] Discontinued Operations [Member]	Sep. 30, 2014 Portfolio Sale, Total [Member] Discontinued Operations [Member]	Sep. 30, 2013 Portfolio Sale, Total [Member] Discontinued Operations [Member]	Sep. 30, 2014 Other Properties [Member] Discontinued Operations [Member]	Sep. 30, 2013 Other Properties [Member] Discontinued Operations [Member]	Sep. 30, 2014 Other Properties [Member] Discontinued Operations [Member]	Sep. 30, 2013 Other Properties [Member] Discontinued Operations [Member]	Dec. 31, 2013 Portfolio Sale, first settlement [Member] sqft properties acre	Jan. 31, 2014 Portfolio Sale, Second Settlement [Member]	Jan. 31, 2014 Portfolio Sale, Second Settlement [Member] Other Segments [Member] Maryland [Member] sqft properties acre	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	Sep. 30, 2014 Land, Buildings and Improvements [Member] bldg	Sep. 30, 2014 Land, Buildings and Improvements [Member] bldg	Sep. 30, 2014 Land, Buildings and Improvements [Member] Not Discontinued Operations [Member] bldg	Sep. 30, 2014 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									75,000		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
Acres of land sold																				140		19
Number of Real Estate Properties Sold																									0	52	0	4
Proceeds from Sale of Property																	126,000,000									366,900,000		37,300,000
Discontinued Operation, Income (Loss) from Discontinued Operation Disclosures [Abstract]
Revenues												9,000	27,585,000	4,728,000	81,385,000
Operating expenses												86,000	(10,606,000)	(2,233,000)	(29,155,000)
Interest and other income												3,000	66,000	40,000	172,000
Interest expense												0	(3,830,000)	(557,000)	(11,550,000)
Depreciation and Amortization												0	(6,798,000)	0	(21,403,000)
Income before (loss) gain on property dispositions												98,000	6,417,000	1,978,000	19,449,000
(Loss) gain on property dispositions	38,000	29,000	46,291,000	49,367,000								(35,000)	0	46,086,000	0
Income from discontinued operations	136,000	6,544,000	48,276,000	70,008,000	136,000	6,544,000	48,276,000	70,008,000				63,000	6,417,000	48,064,000	19,449,000	73,000	127,000	212,000	50,559,000
Noncontrolling Interest												(1,000)	(158,000)	(1,130,000)	(558,000)
Income from discontinued operations available to common shareholders												62,000	6,259,000	46,934,000	18,891,000
Discontinued Operation, Additional Disclosures [Abstract]
Disposal Group, Including Discontinued Operation, Operating Cash Flows												(1,500,000)	11,500,000	(4,300,000)	39,500,000
Disposal Group, Including Discontinued Operations, Investing Cash Flows												$ (42,000)	$ (3,400,000)	$ 313,100,000	$ (11,100,000)