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, 2011 buildings entities	Dec. 31, 2010 buildings
Number of properties held for sale in discontinued operations	3
Number of properties sold in discontinued operations	14	19
Total number of properties in discontinued operations	36
Office [Member]
Number of properties held for sale in discontinued operations	3
Number of properties sold in discontinued operations	11	11
Total number of properties in discontinued operations	25
Industrial [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	3	6
Total number of properties in discontinued operations	9
Retail [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	0	2
Total number of properties in discontinued operations	2