Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share
Outstanding stock options (in shares)	0		0
Stock options, considered antidilutive (in shares)			0
Distributed earnings allocated to common stock	$ 4,146	$ 4,127	$ 12,429	$ 11,899
Undistributed earnings allocated to common stock	10,552	8,706	28,271	24,872
Net earnings allocated to common stock	14,698	12,833	40,700	36,771
Net earnings allocated to participating securities	198	172	542	516
Net income allocated to common stock and participating securities	$ 14,896	$ 13,005	$ 41,242	$ 37,287
Weighted average shares outstanding for basic earnings per common share	24,366,220	24,279,178	24,352,073	24,267,535
Dilutive effect of stock compensation (in shares)	889	10,317	781	10,625
Weighted average shares outstanding for diluted earnings per common share	24,367,109	24,289,495	24,352,854	24,278,160
Basic earnings per common share (in dollars per share)	$ 0.60	$ 0.53	$ 1.67	$ 1.52
Diluted earnings per common share (in dollars per share)	$ 0.60	$ 0.53	$ 1.67	$ 1.51