Provision for Impairment of Investment Properties (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended	9 Months Ended						1 Months Ended	9 Months Ended	1 Months Ended	9 Months Ended
	Sep. 30, 2012 property	Sep. 30, 2011 property	Dec. 31, 2011 property	Sep. 30, 2012 Towson, Maryland	Sep. 30, 2012 El Paso, Texas sqft	Sep. 30, 2012 Fresno, California sqft	Sep. 30, 2012 Highland, California sqft	Sep. 30, 2012 McAllen, Texas sqft	Sep. 30, 2012 Ridgecrest, California sqft	Sep. 30, 2012 13 former Mervyns properties property sqft	Sep. 30, 2012 13 former Mervyns properties sqft	Sep. 30, 2012 Dallas, TX property sqft	Sep. 30, 2012 Dallas, TX sqft	Sep. 30, 2012 Phoenix, Arizona sqft	Sep. 30, 2012 Bakersfield, California sqft	Sep. 30, 2011 Winston-Salem, North Carolina sqft	Sep. 30, 2011 Mesa, Arizona sqft	Sep. 30, 2012 Consolidated properties partially sold to unaffiliated third parties	Sep. 30, 2012 Continuing operations	Sep. 30, 2012 Discontinued operations sqft	Sep. 30, 2011 Discontinued operations
Provision for Impairment of Investment Properties
Approximate square footage					75,500	77,400	80,500	78,000	59,000	1,000,400	1,000,400	132,600	132,600	117,600	75,100	501,000	195,000			2,758,800
Provision for impairment of investment properties	$ 23,490	$ 31,752		$ 1,323	$ 2,694	$ 2,488	$ 2,223	$ 2,950	$ 305		$ 1,100		$ 5,528	$ 4,841	$ 38	$ 30,373	$ 1,379	$ 1,323	$ 11,983	$ 11,507	$ 31,752
Estimated fair value of impaired properties	$ 152,231	$ 19,502
Number of properties sold	20	6	11							13		3