Quarterly Information (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenue	$ 105,985	$ 99,671	$ 102,311	$ 93,605	$ 93,892	$ 88,948	$ 86,034	$ 64,293	$ 401,572	$ 333,167	$ 259,926
Gross profit	19,425	17,929	19,005	15,612	16,266	14,865	14,711	10,792	71,971	56,634	42,690
Net income	$ 6,137	$ 6,183	$ 5,984	$ 4,384	$ 5,940	$ 4,619	$ 4,711	$ 2,590	$ 22,688	$ 17,860	$ 12,732
Earnings per share:
Basic (In Dollar per share)	$ 0.32	$ 0.33	$ 0.32	$ 0.23	$ 0.32	$ 0.25	$ 0.25	$ 0.14	$ 1.20	$ 0.95	$ 0.68
Diluted (In Dollar per share)	$ 0.32	$ 0.32	$ 0.31	$ 0.23	$ 0.31	$ 0.24	$ 0.25	$ 0.14	$ 1.18	$ 0.94	$ 0.68