Quarterly Financial Information (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
Quarterly Financial Information (unaudited)
Revenue	$ 139,146	$ 139,180	$ 136,447	$ 129,619	$ 125,946	$ 123,068	$ 117,886	$ 114,921
Operating income	35,366	34,473	36,103	36,337	33,667	30,799	30,082	30,264	$ 142,279	$ 124,812	$ 94,617
Net income	25,898	25,020	27,279	28,566	27,008	24,288	24,135	25,060	106,763	100,491	81,921
Net income attributable to common shares	$ 19,631	$ 18,600	$ 19,389	$ 20,302	$ 14,912	$ 15,758	$ 15,643	$ 16,292	$ 77,922	$ 62,605	$ 50,371
Net income per share attributable to common shares - basic (in dollars per share)	$ 0.54	$ 0.52	$ 0.57	$ 0.60	$ 0.44	$ 0.47	$ 0.46	$ 0.49	$ 2.23	$ 1.85	$ 1.56
Net income per share attributable to common shares - diluted (in dollars per share)	$ 0.54	$ 0.52	$ 0.57	$ 0.59	$ 0.44	$ 0.46	$ 0.46	$ 0.48	$ 2.22	$ 1.84	$ 1.54