Discontinued Operations and Assets Held for Sale (Table Illustration of Number of Properties in Discontinued Operations) (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2012 buildings	Dec. 31, 2011 buildings
Number of properties sold in discontinued operations	21	101
Total number of properties in discontinued operations	122
Office [Member]
Number of properties sold in discontinued operations	9	93
Total number of properties in discontinued operations	102
Industrial [Member]
Number of properties sold in discontinued operations	11	7
Total number of properties in discontinued operations	18
Retail [Member]
Number of properties sold in discontinued operations	1	1
Total number of properties in discontinued operations	2