Earnings Per Share (Details) (USD $) In Thousands, except 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
Numerator:
Net Earnings	$ 13,844	$ 8,745	$ 13,671	$ 5,324	$ 11,264	$ 9,728	$ 5,855	$ 5,866	$ 41,584	$ 32,713	$ 34,803
Weighted Average Number of Shares Outstanding, Basic									18,544,896	18,832,693	18,805,494
Effect of dilutive securities - employee stock options (in shares)									557,120	527,735	526,609
Weighted Average Number of Shares Outstanding, Diluted									19,102,016	19,360,428	19,332,103
Earnings Per Share, Basic	$ 0.75	$ 0.47	$ 0.74	$ 0.28	$ 0.60	$ 0.52	$ 0.31	$ 0.31	$ 2.24	$ 1.74	$ 1.85
Earnings Per Share, Diluted	$ 0.73	$ 0.46	$ 0.71	$ 0.28	$ 0.59	$ 0.50	$ 0.30	$ 0.30	$ 2.18	$ 1.69	$ 1.80
Anti-dilutive securities excluded from earnings per share calculation (in shares)									233,655	123,292	264,447