Quarterly Information (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information [Line Items]
Revenue	$ 117,220	$ 113,197	$ 104,899	$ 101,373	$ 105,985	$ 99,671	$ 102,311	$ 93,605	$ 436,689	$ 401,572	$ 333,167
Gross profit	21,628	20,061	18,395	16,181	19,425	17,929	19,005	15,612	76,265	71,971	56,634
Net income	$ 7,441	$ 6,143	$ 5,247	$ 4,925	$ 6,137	$ 6,183	$ 5,984	$ 4,384	$ 23,756	$ 22,688	$ 17,860
Earnings per share:
Basic (in dollars per share)	$ 0.39	$ 0.32	$ 0.27	$ 0.26	$ 0.32	$ 0.33	$ 0.32	$ 0.23	$ 1.24	$ 1.20	$ 0.95
Diluted (in dollars per share)	$ 0.38	$ 0.32	$ 0.27	$ 0.26	$ 0.32	$ 0.32	$ 0.31	$ 0.23	$ 1.23	$ 1.18	$ 0.94