Selected Quarterly Financial Data (unaudited) (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
Summary of quarterly financial information
Revenues from continuing operations	$ 107,731	$ 106,953	$ 105,245	$ 104,569	$ 105,309	$ 103,659	$ 102,642	$ 100,021	$ 424,499	$ 411,630	$ 388,471
Income (loss) from continuing operations	(37,151)	3,165	25,224	31,858	52,580	11,857	12,157	9,163	23,096	85,756	58,836
Income (loss) from discontinued operations	0	8	0	(13)	(143)	(23)	(12)	(7)	(4)	(185)	10,600
NET INCOME	$ (37,151)	$ 3,173	$ 25,224	$ 31,845	$ 52,437	$ 11,834	$ 12,145	$ 9,156	$ 20,943	$ 85,571	$ 69,436
Net income attributable to common stockholders per share:
Basic earnings per common share (dollars per share)	$ (0.31)	$ 0.02	$ 0.22	$ 0.28	$ 0.46	$ 0.10	$ 0.12	$ 0.09	$ 0.18	$ 0.79	$ 0.70
Diluted earnings per common share (dollars per share)	$ (0.31)	$ 0.02	$ 0.22	$ 0.28	$ 0.45	$ 0.10	$ 0.12	$ 0.09	$ 0.18	$ 0.78	$ 0.70