Discontinued Operations and Assets Held for Sale (Table Illustration of Number of Properties in Discontinued Operations) (Details)	12 Months Ended
	Dec. 31, 2012 buildings	Dec. 31, 2011 buildings	Dec. 31, 2010 buildings
Number of properties held for sale in discontinued operations	2
Number of properties sold in discontinued operations	28	101	19
Total number of properties in discontinued operations	150
Office [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	10	93	11
Total number of properties in discontinued operations	114
Industrial [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	17	7	6
Total number of properties in discontinued operations	30
Medical Office [Member]
Number of properties held for sale in discontinued operations	2
Number of properties sold in discontinued operations	0	0	0
Total number of properties in discontinued operations	2
Retail [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	1	1	2
Total number of properties in discontinued operations	4