Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of quarterly financial information
Revenues from continuing operations	$ 105,309	$ 103,659	$ 102,642	$ 100,021	$ 98,582	$ 96,725	$ 96,708	$ 96,456	$ 411,630	$ 388,471	$ 370,855
Income from continuing operations	52,580	11,857	12,157	9,163	19,354	16,848	17,586	5,049	85,756	58,836	33,979
Income (loss) from discontinued operations	(143)	(23)	(12)	(7)	(696)	10,632	330	333	(185)	10,600	(1,779)
NET INCOME	52,437	11,834	12,145	9,156	18,658	27,480	17,916	5,382	85,571	69,436	32,200
Less: Net income attributable to noncontrolling interests	0	0	0	0	0	0	0	0			(313)
NET INCOME	$ 52,437	$ 11,834	$ 12,145	$ 9,156	$ 18,658	$ 27,480	$ 17,916	$ 5,382	$ 85,571	$ 69,436	$ 31,887
Net income attributable to common stockholders per share:
Basic earnings per common share (dollars per share)	$ 0.46	$ 0.10	$ 0.12	$ 0.09	$ 0.19	$ 0.28	$ 0.18	$ 0.05	$ 0.79	$ 0.70	$ 0.33
Diluted earnings per common share (dollars per share)	$ 0.45	$ 0.10	$ 0.12	$ 0.09	$ 0.19	$ 0.27	$ 0.18	$ 0.05	$ 0.78	$ 0.70	$ 0.33