Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share [Abstract]
Distributed earnings allocated to common stock	$ 5,150	$ 5,015	$ 15,451	$ 15,045
Undistributed earnings allocated to common stock	5,373	6,479	12,375	15,756
Net earnings allocated to common shareholders	$ 10,523	$ 11,494	$ 27,826	$ 30,801
Average common shares outstanding	14,751	15,003	14,700	15,165
Effect of dilutive securities, Employee stock options	83	68	83	77
Shares for diluted earnings per share	14,834	15,071	14,783	15,242
Basic earnings per share	$ 0.71	$ 0.77	$ 1.89	$ 2.03
Diluted earnings per share	$ 0.71	$ 0.76	$ 1.88	$ 2.02
Options to purchase shares of common stock, anti-dilutive	124,000	255,000
Common stock exercise price, lower limit	$ 35.09	$ 30.38
Common stock exercise price, upper limit	$ 40.88	$ 40.88