Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Earnings Per Share [Abstract]
Distributed earnings allocated to common stock	$ 5,146	$ 5,092	$ 10,291	$ 10,184
Undistributed earnings allocated to common stock	2,208	4,669	7,011	9,123
Net earnings allocated to common shareholders	$ 7,354	$ 9,761	$ 17,302	$ 19,307
Average common shares outstanding	14,680,000	15,120,000	14,676,000	15,244,000
Effect of dilutive securities, Employee stock options	79,000	73,000	84,000	78,000
Shares for diluted earnings per share	14,759,000	15,193,000	14,760,000	15,322,000
Basic earnings per share	$ 0.50	$ 0.65	$ 1.18	$ 1.27
Diluted earnings per share	$ 0.50	$ 0.64	$ 1.17	$ 1.26
Options to purchase shares of common stock, anti-dilutive	208,900	133,000
Common stock exercise price, lower limit	$ 32.93	$ 33.54
Common stock exercise price, upper limit	$ 40.88	$ 40.88