Discontinued Operations and Assets Held for Sale (Table Illustration of Number of Properties in Discontinued Operations) (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2013 buildings	Dec. 31, 2012 buildings
Number of properties held for sale in discontinued operations	1
Number of properties sold in discontinued operations	9	28
Total number of properties in discontinued operations	38
Office [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	2	10
Total number of properties in discontinued operations	12
Industrial [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	4	17
Total number of properties in discontinued operations	21
Medical Office [Member]
Number of properties held for sale in discontinued operations	1
Number of properties sold in discontinued operations	2	0
Total number of properties in discontinued operations	3
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