Consolidated Statements of Changes in Shareholders’ Equity (USD $) In Thousands	Common Stock [Member]	Accumulated Deferred Compensation [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2009	$ 366,247	$ 2,485	$ 3,714	$ 133,002	$ 505,448
Balance (in Shares) at Dec. 31, 2009	29,208
Net income for the year				94,577	94,577
Other comprehensive income			(3,555)		(3,555)
Exercise of stock options	16,688				16,688
Exercise of stock options (in Shares)	406
Tax benefits increase (decrease) upon exercise of stock option	1,004				1,004
Restricted stock activity	194	239			433
Restricted stock activity (in Shares)	7
Stock based compensation	1,380				1,380
Stock awarded to employees	125				125
Stock awarded to employees (in Shares)	2
Purchase and retirement of stock	(6,753)			(21,966)	(28,719)
Purchase and retirement of stock (in Shares)	(533)
Dividends				(42,094)	(42,094)
Balance at Dec. 31, 2010	378,885	2,724	159	163,519	545,287
Balance (in Shares) at Dec. 31, 2010	29,090
Net income for the year				87,888	87,888
Other comprehensive income			11,210		11,210
Exercise of stock options	14,374				14,374
Exercise of stock options (in Shares)	360
Tax benefits increase (decrease) upon exercise of stock option	(248)				(248)
Restricted stock activity	455	336			791
Restricted stock activity (in Shares)	15
Stock based compensation	1,425				1,425
Stock awarded to employees	89				89
Stock awarded to employees (in Shares)	2
Purchase and retirement of stock	(17,205)			(43,300)	(60,505)
Purchase and retirement of stock (in Shares)	(1,317)
Dividends				(41,670)	(41,670)
Balance at Dec. 31, 2011	377,775	3,060	11,369	166,437	558,641
Balance (in Shares) at Dec. 31, 2011	28,150
Net income for the year				81,127	81,127
Other comprehensive income			3,256		3,256
Exercise of stock options	7,635				7,635
Exercise of stock options (in Shares)	185				185
Tax benefits increase (decrease) upon exercise of stock option	(119)				(119)
Restricted stock activity	482	41			523
Restricted stock activity (in Shares)	11
Stock based compensation	1,450				1,450
Stock awarded to employees	93				93
Stock awarded to employees (in Shares)	2
Purchase and retirement of stock	(15,304)			(36,195)	(51,499)
Purchase and retirement of stock (in Shares)	(1,135)
Dividends				(41,005)	(41,005)
Balance at Dec. 31, 2012	$ 372,012	$ 3,101	$ 14,625	$ 170,364	$ 560,102
Balance (in Shares) at Dec. 31, 2012	27,213