Selected Quarterly Financial Data (Unaudited)- Quarterly Financial Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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]
Revenue	$ 396,636	$ 373,918	$ 366,717	$ 341,254	$ 377,910	$ 332,968	$ 324,315	$ 302,780	$ 1,478,525	$ 1,337,973	$ 1,229,162
Operating income	28,298	28,664	22,542	21,889	29,443	24,222	21,025	17,582	101,393	92,272	82,418
Net income	$ 19,379	$ 19,630	$ 14,611	$ 15,318	$ 18,695	$ 16,671	$ 13,617	$ 12,417	$ 68,938	$ 61,400	$ 62,876
Earnings per share:
Basic	$ 1.03	$ 1.04	$ 0.78	$ 0.81	$ 0.99	$ 0.88	$ 0.72	$ 0.67	$ 3.66	$ 3.27	$ 3.35
Diluted	$ 1.01	$ 1.02	$ 0.76	$ 0.80	$ 0.97	$ 0.86	$ 0.71	$ 0.65	$ 3.59	$ 3.18	$ 3.27
Weighted-average common shares outstanding
Basic	18,834	18,799	18,805	18,825	18,838	18,873	18,806	18,670	18,816	18,797	18,766
Diluted	19,234	19,169	19,133	19,263	19,333	19,306	19,209	19,158	19,224	19,335	19,244
Cash dividends declared per common share	$ 0.14	$ 0.14	$ 0.14	$ 0.14	$ 0.14	$ 0.14	$ 0.14	$ 0.14	$ 0.56	$ 0.56