Net Income per Share and Common Stock Dividends (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, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net Income per Share and Common Stock Dividends
Net income, basic and diluted	$ 6,514	$ 6,682	$ 6,103	$ 6,502	$ 6,342	$ 5,774	$ 5,997	$ 5,491	$ 6,051	$ 6,365	$ 5,556	$ 4,981	$ 25,801	$ 23,604	$ 22,953
Average shares outstanding													13,875	13,786	13,689
Effect of dilutive securities (in shares)													57	48	90
Average shares outstanding including dilutive securities													13,932	13,834	13,779
Net income per share, basic (in dollars per share)	$ 0.47	$ 0.48	$ 0.44	$ 0.47	$ 0.46	$ 0.42	$ 0.43	$ 0.40	$ 0.44	$ 0.46	$ 0.41	$ 0.37	$ 1.86	$ 1.71	$ 1.68
Net income per share, diluted (in dollars per share)	$ 0.47	$ 0.48	$ 0.44	$ 0.47	$ 0.46	$ 0.42	$ 0.43	$ 0.40	$ 0.44	$ 0.46	$ 0.40	$ 0.36	$ 1.85	$ 1.71	$ 1.67