Net Income Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net Income Per Share
Net income	$ 7,682	$ 6,682	$ 20,857	$ 19,287
Average shares outstanding	14,408	13,883	14,144	13,867
Dilutive securities	148	83	84	62
Average shares outstanding including dilutive securities	14,556	13,966	14,228	13,929
Net income per share, basic (in dollar per share)	$ 0.53	$ 0.48	$ 1.47	$ 1.39
Net income per share, diluted (in dollar per share)	$ 0.53	$ 0.48	$ 1.47	$ 1.38