Accounting for the Impairment or Disposal of Long-Lived Assets (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Impaired Long-Lived Assets Held and Used [Line Items]
Proceeds from dispositions of operating properties	$ 70,600,000	$ 3,200,000	$ 340,200,000	$ 17,500,000
Impairment charges	0	782,000		1,200,000
South Region [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Number of properties held for sale	3		3
Property Square Feet Held For Sale	484,000		484,000
Impairment charges		201,000		201,000
Central Region [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges		443,000	4,700,000	443,000
Metro [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges		$ 138,000		$ 538,000