Accounting for the Impairment or Disposal of Long-Lived Assets - Narrative (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Impaired Long-Lived Assets Held and Used [Line Items]
Proceeds from dispositions of operating properties		$ 365,200,000	$ 29,000,000	$ 165,600,000
Impairment charges	3,200,000	7,900,000	579,000	5,800,000
Equity Method Investment, Other than Temporary Impairment				94,500,000
Asset Impairment Charges including amounts in Discontinued Operations		7,829,000	957,000	9,509,000
Other Asset Impairment Charges			378,000	3,700,000
Northeast Other [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Number of properties held for sale	7	7
Property Square Feet Held For Sale	544,000	544,000
Asset Impairment Charges including amounts in Discontinued Operations		538,000	0	1,138,000
Central Region [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Number of properties held for sale	18	18
Property Square Feet Held For Sale	1,000,000	1,000,000
Asset Impairment Charges including amounts in Discontinued Operations		5,990,000	511,000	837,000
South Region [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Number of properties held for sale	17	17
Property Square Feet Held For Sale	1,000,000	1,000,000
Asset Impairment Charges including amounts in Discontinued Operations		$ 1,331,000	$ 121,000	$ 4,284,000