Quarterly Financial Summary (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 85,912	$ 76,291	$ 80,263	$ 85,444	$ 91,297	$ 84,708	$ 85,463	$ 88,553	$ 327,910	$ 350,020	$ 329,479
Income before income taxes	15,759	10,549	11,607	14,481	18,709	14,745	15,933	16,806	52,396	66,193	67,746
Net income	$ 9,791	$ 6,757	$ 7,073	$ 8,793	$ 11,797	$ 8,842	$ 9,802	$ 10,436	$ 32,414	$ 40,877	$ 42,034
Net income per common share:
Basic (in dollars per share)	$ 0.61	$ 0.41	$ 0.42	$ 0.51	$ 0.69	$ 0.51	$ 0.56	$ 0.59	$ 1.94	$ 2.36	$ 2.38
Diluted (in dollars per share)	$ 0.60	$ 0.41	$ 0.42	$ 0.51	$ 0.68	$ 0.51	$ 0.56	$ 0.59	$ 1.93	$ 2.33	$ 2.35