Consolidated Statements of Changes in Shareholders’ Equity (unaudited) (USD $) In Thousands	Common Stock [Member]	Accumulated Deferred Compensation [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Total
Beginning balance at Dec. 31, 2011	$ 377,775	$ 3,060	$ 11,369	$ 166,437	$ 558,641
Beginning balance (in Shares) at Dec. 31, 2011	28,150
Net income for the period				21,005	21,005
Other comprehensive income			811		811
Exercise of stock options	641				641
Exercise of stock options (in Shares)	15
Tax benefit increase (decrease) upon exercise of stock options	4				4
Restricted stock activity	220	(220)
Stock based compensation	435				435
Stock awarded to employees	45				45
Stock awarded to employees (in Shares)	1
Purchase and retirement of stock	(3,370)			(8,290)	(11,660)
Purchase and retirement of stock (in Shares)	(249)
Dividends				(10,394)	(10,394)
Ending balance at Mar. 31, 2012	375,750	2,840	12,180	168,758	559,528
Ending balance (in Shares) at Mar. 31, 2012	27,917
Beginning balance at Dec. 31, 2012	372,012	3,101	14,625	170,364	560,102
Beginning balance (in Shares) at Dec. 31, 2012	27,213
Net income for the period				17,271	17,271
Other comprehensive income			747		747
Exercise of stock options	6,156				6,156
Exercise of stock options (in Shares)	151
Tax benefit increase (decrease) upon exercise of stock options	(191)				(191)
Stock based compensation	379				379
Stock awarded to employees	42				42
Stock awarded to employees (in Shares)	1
Purchase and retirement of stock	(4,819)			(10,623)	(15,442)
Purchase and retirement of stock (in Shares)	(347)
Dividends				(10,084)	(10,084)
Ending balance at Mar. 31, 2013	$ 373,579	$ 3,101	$ 15,372	$ 166,928	$ 558,980
Ending balance (in Shares) at Mar. 31, 2013	27,018