Selected Unaudited Quarterly Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Unaudited Quarterly Information
Revenues	$ 66,835	$ 68,626	$ 68,371	$ 68,756	$ 65,988	$ 63,280	$ 60,807	$ 59,813	$ 272,588	$ 249,888	$ 243,867
Income (loss) from continuing operations	(4,932)	1,903	(17,395)	4,480	1,729	2,458	1,612	2,579
Net income (loss)	$ (4,932)	$ 1,903	$ (17,395)	$ 4,480	$ 1,729	$ 2,458	$ 1,612	$ 2,579	$ (15,944)	$ 8,378	$ 35,014
Basic and diluted net income (loss) per share	$ (0.05)	$ 0.02	$ (0.16)	$ 0.04	$ 0.02	$ 0.02	$ 0.02	$ 0.03	$ (0.15)	$ 0.08	$ 0.35
Weighted average number of shares outstanding	107,231,000	107,231,000	107,231,000	107,231,000	107,231,000	103,709,000	100,187,000	100,187,000	107,231,000	102,843,000	100,187,000