Quarterly Financial Summary (unaudited) (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	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 78,085	$ 73,279	$ 72,196	$ 75,688	$ 78,625	$ 73,803	$ 76,745	$ 83,966	$ 299,248	$ 313,139	$ 327,910
Income from operations before income taxes	12,777	7,616	6,333	8,318	11,259	429	10,697	16,007	35,044	38,392	52,396
Net income	$ 8,417	$ 4,366	$ 3,829	$ 4,509	$ 6,893	$ 326	$ 6,596	$ 10,340	$ 21,121	$ 24,155	$ 32,414
Net income per common share:
Basic (in dollars per share)	$ 0.52	$ 0.27	$ 0.24	$ 0.28	$ 0.43	$ 0.02	$ 0.41	$ 0.64	$ 1.30	$ 1.50	$ 1.94
Diluted (in dollars per share)	$ 0.51	$ 0.27	$ 0.23	$ 0.28	$ 0.42	$ 0.02	$ 0.41	$ 0.64	$ 1.29	$ 1.49	$ 1.93