Quarterly Information (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 125,200	$ 123,869	$ 134,918	$ 117,880	$ 117,220	$ 113,197	$ 104,899	$ 101,373	$ 501,867	$ 436,689	$ 401,572
Gross profit	23,935	22,518	24,767	18,355	21,628	20,061	18,395	16,181	89,575	76,265	71,971
Net income	$ 7,424	$ 7,244	$ 8,113	$ 4,317	$ 7,441	$ 6,143	$ 5,247	$ 4,925	$ 27,098	$ 23,756	$ 22,688
Earnings per share:
Basic (in dollars per share)	$ 0.43	$ 0.38	$ 0.42	$ 0.23	$ 0.39	$ 0.32	$ 0.27	$ 0.26	$ 1.45	$ 1.24	$ 1.20
Diluted (in dollars per share)	$ 0.43	$ 0.37	$ 0.42	$ 0.22	$ 0.38	$ 0.32	$ 0.27	$ 0.26	$ 1.43	$ 1.23	$ 1.18