Accounting for the Impairment or Disposal of Long-Lived Assets (Asset Impairments in Reportable Segments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Asset impairment charges	$ 10,632	$ 3,879	$ 18,244
Houston [Member]
Segment Reporting Information [Line Items]
Asset impairment charges	10,632	0	0
Carolinas Richmond [Member]
Segment Reporting Information [Line Items]
Asset impairment charges	0	0	13,755
Minnesota [Member]
Segment Reporting Information [Line Items]
Asset impairment charges	0	3,879	0
Southeastern PA [Member]
Segment Reporting Information [Line Items]
Asset impairment charges	0	0	2,328
Other Segments [Member]
Segment Reporting Information [Line Items]
Asset impairment charges	0	$ 0	2,161
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Asset impairment charges	6,700		1,000
Land and Land Improvements [Member] | impairment - real estate assets [Member]
Segment Reporting Information [Line Items]
Asset impairment charges	$ 3,900		$ 1,000