Discontinued Operations and Assets Held for Sale (Table Illustration of Number of Properties in Discontinued Operations) (Details)	12 Months Ended
	Dec. 31, 2014 buildings	Dec. 31, 2013 buildings	Dec. 31, 2012 buildings
Number of properties held for sale in discontinued operations	1	11
Number of properties sold in discontinued operations	12	25	28
Total number of properties in discontinued operations	66
Continuing Operations, Number of In-service Properties Held for Sale	1
Continuing Operations, Number of Real Estate Properties Sold	17	13	0
Continuing Operations, Number of Real Estate Properties Sold or Classified as Held for Sale	31
Number of Real Estate Properties Held for Sale	2
Number of Real Estate Properties Sold	29	38	28
Total Properties Sold or Classified as Held for Sale	97
Office Building [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	0	12	10
Total number of properties in discontinued operations	22
Industrial [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	11	6	17
Total number of properties in discontinued operations	34
Medical Office [Member]
Number of properties held for sale in discontinued operations	1
Number of properties sold in discontinued operations	1	6	0
Total number of properties in discontinued operations	8
Retail [Member]
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	0	1	1
Total number of properties in discontinued operations	2