Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information (unaudited)
Revenues	$ 146,035	$ 144,891	$ 142,906	$ 138,895	$ 139,146	$ 139,180	$ 136,447	$ 129,619	$ 572,727	$ 544,392	$ 481,821
Operating income	35,684	33,643	36,056	35,433	35,366	34,473	36,103	36,337	140,816	142,279	124,812
Net income	24,745	22,644	25,743	25,905	25,898	25,020	27,279	28,566	99,037	106,763	100,491
Net income attributable to common shares	$ 19,194	$ 17,450	$ 19,535	$ 19,661	$ 19,631	$ 18,600	$ 19,389	$ 20,302	$ 75,840	$ 77,922	$ 62,605
Net income per share attributable to common shares - basic (in dollars per share)	$ 0.51	$ 0.47	$ 0.54	$ 0.54	$ 0.54	$ 0.52	$ 0.57	$ 0.60	$ 2.06	$ 2.23	$ 1.85
Net income per share attributable to common shares - diluted (in dollars per share)	$ 0.51	$ 0.47	$ 0.53	$ 0.54	$ 0.54	$ 0.52	$ 0.57	$ 0.59	$ 2.05	$ 2.22	$ 1.84