Quarterly Results (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 Disclosure [Abstract]
Total revenue	$ 160,997	$ 155,152	$ 158,914	$ 158,843	$ 161,484	$ 158,731	$ 156,501	$ 154,138	$ 633,906	$ 630,854	$ 611,991
Operating income	22,880	25,359	24,020	24,787	26,994	28,329	21,169	16,754	97,046	93,246	46,510
Net income	$ 13,521	$ 14,354	$ 13,150	$ 13,910	$ 14,852	$ 15,534	$ 9,626	$ 6,583	$ 54,935	$ 46,595	$ 66,390
Net income per share - basic (in dollars per share)	$ 0.27	$ 0.29	$ 0.26	$ 0.28	$ 0.31	$ 0.31	$ 0.19	$ 0.13	$ 1.10	$ 0.94	$ 1.35
Net income per share - diluted (in dollars per share)	$ 0.27	$ 0.29	$ 0.26	$ 0.28	$ 0.30	$ 0.31	$ 0.19	$ 0.13	$ 1.10	$ 0.93	$ 1.33