Accounting for the Impairment or Disposal of Long-Lived Assets - Discontinued Operations (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 30, 2013 sqft properties	Jan. 31, 2014
Discontinued Operation, Income (Loss) from Discontinued Operation Disclosures [Abstract]
Gain on property dispositions									$ 46,631,000	$ 95,384,000	$ 12,426,000
Income from discontinued operations	306,000	133,000	296,000	47,846,000	51,832,000	6,544,000	13,633,000	49,830,000	48,581,000	121,839,000	51,004,000
Discontinued Operations [Member]
Discontinued Operation, Income (Loss) from Discontinued Operation Disclosures [Abstract]
Revenues									4,728,000	113,586,000	137,476,000
Operating expenses									(2,258,000)	(41,875,000)	(48,956,000)
Interest and other income									37,000	213,000	580,000
Interest expense									(557,000)	(15,903,000)	(19,783,000)
Depreciation and amortization									0	(29,566,000)	(30,739,000)
Income before property dispositions									1,950,000	26,455,000	38,578,000
Gain on property dispositions									46,631,000	95,384,000	12,426,000
Income from discontinued operations									48,581,000	121,839,000	51,004,000
Other Segments [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties	1								1
Square Footage of Real Estate Property	83,000								83,000
Portfolio Sale, Total [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties												97
Square Footage of Real Estate Property												6,600,000
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds												697,300,000
Number of Real Estate Settlements					2					2
Area of Land					159					159
Portfolio Sale, first settlement [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties					49					49
Square Footage of Real Estate Property					4,000,000					4,000,000
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds					367,700,000					367,700,000
Area of Land					140					140
Portfolio Sale, Second Settlement [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds													329,600,000
Portfolio Sale, Second Settlement [Member] | Other Segments [Member] | Maryland [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties													23
Square Footage of Real Estate Property													1,400,000
Area of Land													19
Portfolio Sale, Second Settlement [Member] | Other Segments [Member] | New Jersey [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties													24
Square Footage of Real Estate Property													1,200,000
Portfolio Sale, Second Settlement [Member] | Southeastern PA [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties													1
Square Footage of Real Estate Property													37,000
Other Properties [Member] | Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Property, Plant, and Equipment										$ 136,700,000	$ 228,500,000