Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Summary of quarterly financial information
Revenues from continuing operations	$ 90,116	[1]	$ 84,311	[2]	$ 82,062	[3]	$ 80,437	[4]	$ 78,250	[5]	$ 76,481	[6]	$ 75,487	[7]	$ 73,856	[8]	$ 336,926	$ 304,074	$ 278,598
Income (loss) from continuing operations	9,888	[1]	4,703	[2]	(27,701)	[3]	842	[4]	(1,467)	[5]	635	[6]	875	[7]	52	[8]	(12,268)	95	(11,543)
Discontinued operations	2,568	[1]	15,080	[2]	3,463	[3]	(1,860)	[4]	(4,895)	[5]	5,200	[6]	2,053	[7]	3,082	[8]	19,251	5,440	11,359
NET INCOME (LOSS)	12,456	[1]	19,783	[2]	(24,238)	[3]	(1,018)	[4]	(6,362)	[5]	5,835	[6]	2,928	[7]	3,134	[8]	6,983	5,535	(184)
Less: Net income attributable to noncontrolling interests	(72)	[1]	(17)	[2]	33	[3]	19	[4]	(29)	[5]	(21)	[6]	(20)	[7]	0	[8]	(37)	(70)	(30)
NET INCOME (LOSS) ATTRIBUTABLE TO COMMON STOCKHOLDERS	$ 12,384	[1]	$ 19,766	[2]	$ (24,205)	[3]	$ (999)	[4]	$ (6,391)	[5]	$ 5,814	[6]	$ 2,908	[7]	$ 3,134	[8]	$ 6,946	$ 5,465	$ (214)
Net income attributable to common stockholders per share:
Basic earnings (loss) per common share (dollars per share)	$ 0.13	[1]	$ 0.21	[2]	$ (0.27)	[3]	$ (0.01)	[4]	$ (0.07)	[5]	$ 0.08	[6]	$ 0.04	[7]	$ 0.04	[8]	$ 0.08	$ 0.07	$ 0.00
Diluted earnings (loss) per common share (dollars per share)	$ 0.13	[1]	$ 0.21	[2]	$ (0.27)	[3]	$ (0.01)	[4]	$ (0.07)	[5]	$ 0.07	[6]	$ 0.04	[7]	$ 0.04	[8]	$ 0.08	$ 0.07	$ 0.00

[1] The increases in net income and amounts per share for the fourth quarter of 2013 are primarily attributable to gains on sales of real estate totaling $2.7 million.

[2] The increases in net income and amounts per share for the third quarter of 2013 are primarily attributable to gains on sales of real estate totaling $20.2 million, partially offset by impairment charges of $6.3 million.

[3] The decreases in net income and amounts per share for the second quarter of 2013 are primarily attributable to losses of extinguishment of debt of $29.9 million, of which $0.3 million is included in discontinued operations. This amount is partially offset by gains on sales of real estate totaling $1.8 million.

[4] The decreases in net income and amounts per share for the first quarter of 2013 are primarily attributable to impairment charges of $3.6 million.

[5] The decreases in net income and amounts per share for the fourth quarter of 2012 are primarily attributable to impairment charges of $7.7 million. This amount is partially offset by gains on sales of real estate totaling $1.2 million.

[6] The increases in net income and amounts per share for the third quarter of 2012 are primarily attributable to gains on sales of real estate totaling $6.3 million, partially offset by impairment charges of $2.9 million.

[7] The decreases in net income and amounts per share for the second quarter of 2012 are primarily attributable to impairment charges of $0.2 million.

[8] The increases in net income and amounts per share for the first quarter of 2012 are primarily attributable to gains on sales of real estate totaling $3.4 million and a lease termination fee totaling $1.5 million which is included in discontinued operations. This amount is partially offset by impairment charges of $4.2 million.