Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2015	[1]	Sep. 30, 2015	[2]	Jun. 30, 2015	[3]	Mar. 31, 2015	[4]	Dec. 31, 2014	[5]	Sep. 30, 2014	[6]	Jun. 30, 2014	[7]	Mar. 31, 2014	[8]	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of quarterly financial information
Revenues from continuing operations	$ 98,582		$ 96,725		$ 96,708		$ 96,456		$ 95,001		$ 93,612		$ 91,671		$ 90,571		$ 388,471	$ 370,855	$ 330,949
Income from continuing operations	19,354		16,848		17,586		5,049		9,060		8,437		9,005		7,477		58,836	33,979	(13,092)
Discontinued operations	(696)		10,632		330		333		9,013		(4,284)		(2,994)		(3,514)		10,600	(1,779)	20,075
NET INCOME	18,658		27,480		17,916		5,382		18,073		4,153		6,011		3,963		69,436	32,200	6,983
Less: Net income attributable to noncontrolling interests	0		0		0		0		0		(162)		(40)		(111)			(313)	(37)
NET INCOME (LOSS) ATTRIBUTABLE TO COMMON STOCKHOLDERS	$ 18,658		$ 27,480		$ 17,916		$ 5,382		$ 18,073		$ 3,991		$ 5,971		$ 3,852		$ 69,436	$ 31,887	$ 6,946
Net income attributable to common stockholders per share:
Basic earnings per common share (dollars per share)	$ 0.19		$ 0.28		$ 0.18		$ 0.05		$ 0.19		$ 0.04		$ 0.06		$ 0.04		$ 0.70	$ 0.33	$ 0.08
Diluted earnings per common share (dollars per share)	$ 0.19		$ 0.27		$ 0.18		$ 0.05		$ 0.18		$ 0.04		$ 0.06		$ 0.04		$ 0.70	$ 0.33	$ 0.08

[1] The increases in net income and amounts per share for the fourth quarter of 2015 are primarily attributable to gains on sales of real estate totaling $9.1 million, partially offset by impairment charges of $0.7 million.

[2] The increases in net income and amounts per share for the third quarter of 2015 are primarily attributable to a $10.6 million gain on the sale of one property previously classified as an asset held for sale.

[3] The increases in net income and amounts per share for the second quarter of 2015 are primarily attributable to a $41.5 million gain on the sale of two properties, partially offset by a $28.0 million loss on extinguishment of the Senior Notes due 2017.

[4] The decreases in net income and amounts per share for the first quarter of 2015 are primarily attributable to impairment charges of $3.3 million

[5] The increases in net income and amounts per share for the fourth quarter of 2014 are primarily attributable to gains on sales of real estate totaling $9.3 million, partially offset by impairment charges of $1.0 million.

[6] The decreases in net income and amounts per share for the third quarter of 2014 are primarily attributable to impairment charges of $4.5 million.

[7] The increases in net income and amounts per share for the second quarter of 2014 are primarily attributable to a $1.9 million cash reimbursement received by the Company for certain operating expenses paid by the Company for years 2006 through 2013.

[8] The decreases in net income and amounts per share for the first quarter of 2014 are primarily attributable to impairment charges of $3.4 million.