Accounting for the Impairment or Disposal of Long-Lived Assets (Discontinued Operations) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Discontinued Operation
Gain on property dispositions									$ 14,578	$ 0	$ 0
Net income	$ 17,361	$ (2,809)	$ 2,308	$ 2,119	$ 3,253	$ 4,059	$ 2,242	$ 2,437	18,979	11,991	10,565
Discontinued operations net of noncontrolling interest									18,535	11,709	10,319
Discontinued Operations [Member]
Discontinued Operation
Revenues									34,035	36,010	34,813
Operating expenses									(14,014)	(12,989)	(12,309)
Interest and other income									58	133	23
Interest expense									(2,239)	(2,778)	(2,222)
Depreciation and amortization									(6,753)	(8,385)	(8,711)
Impairment charges - real estate assets									(6,686)	0	(1,029)
Net income before property dispositions									4,401	11,991	10,565
Gain on property dispositions									14,578	0	0
Net income									18,979	11,991	10,565
Noncontrolling interest - operating partnership									(444)	(282)	(246)
Discontinued operations net of noncontrolling interest									18,535	11,709	10,319
Capital Expenditure, Discontinued Operations									$ 8,100	$ 1,500	$ 9,900