Discontinued Operations and Assets Held for Sale (Table Illustration of Number of Properties in Discontinued Operations) (Details)	12 Months Ended
	Dec. 31, 2011 buildings	Dec. 31, 2010 buildings	Dec. 31, 2009 buildings
Number of properties held for sale in discontinued operations	13
Number of properties sold in discontinued operations	101	19	5
Total number of properties in discontinued operations	138
Office [Member]
Number of properties held for sale in discontinued operations	7
Number of properties sold in discontinued operations	93	11	5
Total number of properties in discontinued operations	116
Industrial [Member]
Number of properties held for sale in discontinued operations	6
Number of properties sold in discontinued operations	7	6	0
Total number of properties in discontinued operations	19
Retail [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	1	2	0
Total number of properties in discontinued operations	3