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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	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, 2013	Dec. 31, 2012	Dec. 31, 2011
Net Income per Share and Common Stock Dividends
Net income, basic and diluted	$ 6,313	$ 7,682	$ 6,407	$ 6,768	$ 6,514	$ 6,682	$ 6,103	$ 6,502	$ 6,342	$ 5,774	$ 5,997	$ 5,491	$ 27,170	$ 25,801	$ 23,604
Average shares outstanding													14,223	13,875	13,786
Effect of dilutive securities (in shares)													130	57	48
Average shares outstanding including dilutive securities													14,353	13,932	13,834
Net income per share, basic (in dollars per share)	$ 0.44	$ 0.53	$ 0.45	$ 0.49	$ 0.47	$ 0.48	$ 0.44	$ 0.47	$ 0.46	$ 0.42	$ 0.43	$ 0.40	$ 1.91	$ 1.86	$ 1.71
Net income per share, diluted (in dollars per share)	$ 0.43	$ 0.53	$ 0.45	$ 0.49	$ 0.47	$ 0.48	$ 0.44	$ 0.47	$ 0.46	$ 0.42	$ 0.43	$ 0.40	$ 1.89	$ 1.85	$ 1.71