Discontinued Operations and Assets Held for Sale (Table Illustration of Number of Properties in Discontinued Operations) (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2012 buildings	Dec. 31, 2011 buildings
Number of properties held for sale in discontinued operations	3
Number of properties sold in discontinued operations	13	101
Total number of properties in discontinued operations	117
Office [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	7	93
Total number of properties in discontinued operations	100
Industrial [Member]
Number of properties held for sale in discontinued operations	2
Number of properties sold in discontinued operations	6	7
Total number of properties in discontinued operations	15
Retail [Member]
Number of properties held for sale in discontinued operations	1
Number of properties sold in discontinued operations	0	1
Total number of properties in discontinued operations	2