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, 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
Beginning balance at Dec. 31, 2013	378,946	2,711	4,313	156,964	542,934
Beginning balance (in Shares) at Dec. 31, 2013	26,510
Net income for the period				15,307	15,307
Other comprehensive income			4,543		4,543
Exercise of stock options	10,853				10,853
Exercise of stock options (in Shares)	225
Tax benefit increase (decrease) upon exercise of stock options	(369)				(369)
Stock based compensation	359				359
Stock awarded to employees	52				52
Stock awarded to employees (in Shares)	1
Purchase and retirement of stock	(6,351)			(16,359)	(22,710)
Purchase and retirement of stock (in Shares)	(437)
Dividends				(10,086)	(10,086)
Ending balance at Mar. 31, 2014	$ 383,490	$ 2,711	$ 8,856	$ 145,826	$ 540,883
Ending balance (in Shares) at Mar. 31, 2014	26,299