Accounting for the Impairment or Disposal of Long-Lived Assets (Discontinued Operations by Reportable Segment) (Details) - bldg	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	60
Discontinued Operations, Held-for-sale
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	10	27
Discontinued Operations, Disposed of by Sale
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	37	2
Florida | Discontinued Operations, Held-for-sale
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	3
Florida | Discontinued Operations, Disposed of by Sale
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	0	0
Houston | Discontinued Operations, Held-for-sale
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	0
Houston | Discontinued Operations, Disposed of by Sale
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	0	1
Southeastern PA | Discontinued Operations, Held-for-sale
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	5
Southeastern PA | Discontinued Operations, Disposed of by Sale
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	32	1
Other | Discontinued Operations, Held-for-sale
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	2
Other | Discontinued Operations, Disposed of by Sale
Segment Reporting Information [Line Items]
Number of buildings (in buildings)	5	0