Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of quarterly financial information
Revenues from continuing operations	$ 81,260	$ 79,528	$ 78,551	$ 77,011	$ 75,259	$ 74,259	$ 71,358	$ 70,716	$ 316,350	$ 291,592	$ 249,997
Income from continuing operations	395	2,507	2,826	2,084	1,739	46	1,489	(6,200)	7,812	(2,926)	1,060
Discontinued operations	(6,757)	3,328	102	1,050	1,178	604	522	438	(2,277)	2,742	7,187
NET INCOME (LOSS)	(6,362)	5,835	2,928	3,134	2,917	650	2,011	(5,762)	5,535	(184)	8,247
Less: Net income attributable to noncontrolling interests	(30)	(20)	(20)	0	0	(3)	0	(27)	(70)	(30)	(47)
NET INCOME (LOSS) ATTRIBUTABLE TO COMMON STOCKHOLDERS	$ (6,392)	$ 5,815	$ 2,908	$ 3,134	$ 2,917	$ 647	$ 2,011	$ (5,789)	$ 5,465	$ (214)	$ 8,200
Basic Earnings (Loss) Per Common Share
Income (loss) from continuing operations (in dollars per share)	$ 0.00	$ 0.04	$ 0.04	$ 0.03	$ 0.02	$ 0.00	$ 0.02	$ (0.10)	$ 0.10	$ (0.04)	$ 0.02
Discontinued operations (in dollars per share)	$ (0.07)	$ 0.04	$ 0.00	$ 0.01	$ 0.02	$ 0.01	$ 0.01	$ 0.01	$ (0.03)	$ 0.04	$ 0.11
Net income (loss) attributable to common stockholders (in dollars per share)	$ (0.07)	$ 0.08	$ 0.04	$ 0.04	$ 0.04	$ 0.01	$ 0.03	$ (0.09)	$ 0.07		$ 0.13
Diluted Earnings (Loss) Per Common Share
Income (loss) from continuing operations (in dollars per share)	$ 0.00	$ 0.03	$ 0.04	$ 0.03	$ 0.02	$ 0.00	$ 0.02	$ (0.10)	$ 0.10	$ (0.04)	$ 0.02
Discontinued operations	$ (0.07)	$ 0.04	$ 0.00	$ 0.01	$ 0.02	$ 0.01	$ 0.01	$ 0.01	$ (0.03)	$ 0.04	$ 0.11
Net income (loss) attributable to common stockholders (in dollars per share)	$ (0.07)	$ 0.07	$ 0.04	$ 0.04	$ 0.04	$ 0.01	$ 0.03	$ (0.09)	$ 0.07		$ 0.13