Real Estate Assets, Discontinued Operations and Assets Held for Sale (Table Illustration of Number of Properties in Discontinued Operations) (Details) - buildings	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Number of properties held for sale in discontinued operations	0
Number of properties sold in discontinued operations	0	81
Total number of properties in discontinued operations	81
Continuing Operations, Number of In-service Properties Held for Sale	2
Continuing Operations, Number of Real Estate Properties Sold	6	17
Continuing Operations, Number of Real Estate Properties Sold or Classified as Held for Sale	25
Number of Real Estate Properties Held for Sale	2
Number of Real Estate Properties Sold	6	98
Total Properties Sold or Classified as Held for Sale	106