Real Estate Assets, Discontinued Operations and Assets Held for Sale (Table Illustration of Number of Properties in Discontinued Operations) (Details) - buildings	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 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	1
Continuing Operations, Number of Real Estate Properties Sold	13	17
Continuing Operations, Number of Real Estate Properties Sold or Classified as Held for Sale	31
Number of Real Estate Properties Held for Sale	1
Number of Real Estate Properties Sold	13	98
Total Properties Sold or Classified as Held for Sale	112