Held for Sale and Discontinued Operations (Details 1) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues
Rental income									$ 407,481	$ 383,333	$ 361,525
Other operating									4,149	5,047	5,665
Expenses
Property operating									146,458	140,195	134,057
General and administrative									35,805	26,925	22,808
Depreciation									116,483	106,530	99,384
Bad debt, net of recoveries									(21)	(193)	31
Other Income (Expense)
Interest and other income, net									375	389	2,637
Income from discontinued operations									(71)	715	967
Impairments									(121)	(686)	(12,029)
Gain on sales of real estate properties									7	10,571	9,283
INCOME (LOSS) FROM DISCONTINUED OPERATIONS	$ (143)	$ (23)	$ (12)	$ (7)	$ (696)	$ 10,632	$ 330	$ 333	$ (185)	$ 10,600	$ (1,779)
Income (Loss) from Discontinued Operations per Common Share - Basic (in dollars per share)									$ 0.00	$ 0.11	$ (0.02)
Income (Loss) from Discontinued Operations per Common Share - Diluted (in dollars per share)									$ 0.00	$ 0.11	$ (0.02)
Discontinued Operations [Member]
Revenues
Rental income									$ 0	$ 752	$ 5,660
Other operating									0	0	4
Total Revenue									0	752	5,664
Expenses
Property operating									71	58	3,126
General and administrative									0	0	19
Depreciation									0	0	1,551
Bad debt, net of recoveries									0	(1)	3
Total Expense									71	57	4,699
Other Income (Expense)
Interest and other income, net									0	20	2
Total other income (expense)									0	20	2
Income from discontinued operations									(71)	715	967
Impairments									(121)	(686)	(12,029)
Gain on sales of real estate properties									7	10,571	9,283
INCOME (LOSS) FROM DISCONTINUED OPERATIONS									$ (185)	$ 10,600	$ (1,779)
Income (Loss) from Discontinued Operations per Common Share - Basic (in dollars per share)									$ 0.00	$ 0.11	$ (0.02)
Income (Loss) from Discontinued Operations per Common Share - Diluted (in dollars per share)									$ 0.00	$ 0.11	$ (0.02)