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, 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 Disclosure [Abstract]
Revenue	$ 377,910	$ 332,968	$ 324,315	$ 302,780	$ 321,174	$ 305,301	$ 306,392	$ 296,295	$ 1,337,973	$ 1,229,162	$ 1,185,097
Operating income	29,443	24,222	21,025	17,582	20,181	23,396	22,208	16,633	92,272	82,418	82,793
Net income	$ 18,695	$ 16,671	$ 13,617	$ 12,417	$ 27,070	$ 13,692	$ 11,937	$ 10,177	$ 61,400	$ 62,876	$ 46,584
Earnings per share:
Basic	$ 0.99	$ 0.88	$ 0.72	$ 0.67	$ 1.45	$ 0.73	$ 0.64	$ 0.54	$ 3.27	$ 3.35	$ 2.45
Diluted	$ 0.97	$ 0.86	$ 0.71	$ 0.65	$ 1.41	$ 0.72	$ 0.63	$ 0.52	$ 3.18	$ 3.27	$ 2.40
Weighted-average common shares outstanding
Basic	18,838	18,873	18,806	18,670	18,646	18,666	18,775	18,972	18,797	18,766	18,989
Diluted	19,333	19,306	19,209	19,158	19,136	19,024	19,086	19,423	19,335	19,244	19,416
Cash dividends declared per common share	$ 0.14	$ 0.14	$ 0.14	$ 0.14					$ 0.56