Selected Quarterly Financial Data (Unaudited)- Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	[1]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 321,174	$ 305,301	$ 306,392	$ 296,295	$ 289,559	$ 306,520	$ 305,419	$ 283,599		$ 1,229,162	$ 1,185,097	$ 1,132,232
Operating income	20,181	23,396	22,208	16,633	21,965	23,776	19,358	17,694		82,418	82,793	75,231
Net income	$ 27,070	$ 13,692	$ 11,937	$ 10,177	$ 12,673	$ 13,437	$ 10,583	$ 9,891		$ 62,876	$ 46,584	$ 39,369
Earnings per share:
Basic	$ 1.45	$ 0.73	$ 0.64	$ 0.54	$ 0.67	$ 0.71	$ 0.56	$ 0.52		$ 3.35	$ 2.45	$ 2.04
Diluted	$ 1.41	$ 0.72	$ 0.63	$ 0.52	$ 0.65	$ 0.70	$ 0.55	$ 0.51		$ 3.27	$ 2.40	$ 2.00
Weighted-average common shares outstanding (in thousands)
Basic	18,646	18,666	18,775	18,972	18,988	18,965	19,008	18,994		18,766,000	18,989,000	19,335,000
Diluted	19,136	19,024	19,086	19,423	19,512	19,329	19,293	19,273		19,244,000	19,416,000	19,663,000

[1] Includes adjustments for the adoption ASU 2016-09 in the second quarter of 2016.