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, 2013 buildings	Dec. 31, 2012 buildings
Number of properties held for sale in discontinued operations	6
Number of properties sold in discontinued operations	11	28
Total number of properties in discontinued operations	45
Office [Member]
Number of properties held for sale in discontinued operations	4
Number of properties sold in discontinued operations	2	10
Total number of properties in discontinued operations	16
Industrial [Member]
Number of properties held for sale in discontinued operations	1
Number of properties sold in discontinued operations	5	17
Total number of properties in discontinued operations	23
Medical Office [Member]
Number of properties held for sale in discontinued operations	1
Number of properties sold in discontinued operations	3	0
Total number of properties in discontinued operations	4
Retail [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	1	1
Total number of properties in discontinued operations	2