Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of quarterly financial information
Revenues from continuing operations	$ 113,168	$ 113,462	$ 111,634	$ 112,124	$ 107,749	$ 107,025	$ 105,318	$ 104,644	$ 450,389	$ 424,737	$ 411,955
NET INCOME	$ 16,314	$ 6,548	$ 37,729	$ 9,180	$ (37,151)	$ 3,173	$ 25,224	$ 31,845
Net income attributable to common stockholders per share:
Basic earnings per common share (dollars per share)	$ 0.13	$ 0.05	$ 0.30	$ 0.07	$ (0.31)	$ 0.02	$ 0.22	$ 0.28	$ 0.55	$ 0.18	$ 0.79
Diluted earnings per common share (dollars per share)	$ 0.13	$ 0.05	$ 0.30	$ 0.07	$ (0.31)	$ 0.02	$ 0.22	$ 0.28	$ 0.55	$ 0.18	$ 0.78