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
Quarterly financial data (unaudited)
Revenue	$ 28,000	$ 26,017	$ 24,302	$ 24,187	$ 25,899	$ 24,688	$ 22,943	$ 21,028	$ 102,506	$ 94,558	$ 80,420
Income from operations	2,074	4,129	2,236	2,407	4,040	2,981	2,427	1,729	10,846	11,177	7,355
Net (loss) income attributable to common stockholders	1,400	2,777	1,461	1,657	1,855	1,662	1,293	(148)	7,295	4,662	10,554
Basic earnings per share (in dollars per share)	$ 0.04	$ 0.08	$ 0.04	$ 0.05	$ 0.06	$ 0.05	$ 0.06	$ (0.03)	$ 0.21	$ 0.19	$ 1.81
Diluted earnings per share (in dollars per share)	$ 0.04	$ 0.07	$ 0.04	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ (0.03)	$ 0.20	$ 0.17	$ 0.49
Increased net income attributable to common stockholders									$ 243