Quarterly Financial Information (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, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information (unaudited)
Revenue	$ 55,252	$ 53,762	$ 50,636	$ 47,284	$ 46,029	$ 44,367	$ 42,484	$ 39,966	$ 38,352	$ 206,934	$ 172,846
Operating income (loss)	5,936	5,059	3,814	2,234	1,072	1,125	(3,473)	(5,814)	(8,697)	17,043	(7,090)
Net income (loss)	4,578	2,947	1,848	1,343	462	263	(3,588)	(7,916)	(10,722)	10,716	(10,779)	(9,181)
Net income (loss) attributable to common shares	$ 1,862	$ 1,320	$ 826	$ 600	$ 179	$ 112	$ (1,530)	$ (3,372)	$ (3,351)	$ 4,608	$ (4,611)
Net income (loss) per share attributable to common shares - basic (in dollars per share)	$ 0.09	$ 0.06	$ 0.04	$ 0.03	$ 0.01	$ 0.01	$ (0.08)	$ (0.17)	$ (0.17)	$ 0.22	$ (0.24)
Net income (loss) per share attributable to common shares - diluted (in dollars per share)	$ 0.09	$ 0.06	$ 0.04	$ 0.03	$ 0.01	$ 0.01	$ (0.08)	$ (0.17)	$ (0.17)	$ 0.22	$ (0.24)