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				41,969	41,969
Other comprehensive income			1,661		1,661
Exercise of stock options	1,030				1,030
Exercise of stock options (in Shares)	25
Tax benefit increase (decrease) upon exercise of stock options	2				2
Restricted stock activity	482	41			523
Restricted stock activity (in Shares)	11
Stock based compensation	805				805
Stock awarded to employees	63				63
Stock awarded to employees (in Shares)	1
Purchase and retirement of stock	(7,526)			(18,555)	(26,081)
Purchase and retirement of stock (in Shares)	(566)
Dividends				(20,687)	(20,687)
Ending balance at Jun. 30, 2012	372,631	3,101	13,030	169,164	557,926
Ending balance (in Shares) at Jun. 30, 2012	27,621
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				34,383	34,383
Other comprehensive income			(10,101)		(10,101)
Exercise of stock options	6,414				6,414
Exercise of stock options (in Shares)	157
Tax benefit increase (decrease) upon exercise of stock options	(201)				(201)
Restricted stock activity	1,068	(390)			678
Restricted stock activity (in Shares)	15
Stock based compensation	730				730
Stock awarded to employees	61				61
Stock awarded to employees (in Shares)	1
Purchase and retirement of stock	(8,586)			(18,807)	(27,393)
Purchase and retirement of stock (in Shares)	(617)
Dividends				(20,051)	(20,051)
Ending balance at Jun. 30, 2013	$ 371,498	$ 2,711	$ 4,524	$ 165,889	$ 544,622
Ending balance (in Shares) at Jun. 30, 2013	26,769