CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) (USD $) In Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Stockholders' Equity, Beginning Balance at Dec. 31, 2011	$ 281,351	$ 1,018	$ 88,030	$ 194,198	$ (1,895)
Common Stock, Shares Outstanding, Beginning Balance at Dec. 31, 2011		16,292
Net income	26,051			26,051
Total other comprehensive income (loss), net of tax	(202)				(202)
Cash dividends declared	(11,610)			(11,610)
Share-based compensation	1,404		1,404
Deferred compensation plan, shares		10
Deferred compensation plan	146	1	145
Exercise of stock options, issuance of other compensation-related equity instruments and related tax benefit, shares		69
Exercise of stock options, issuance of other compensation-related equity instruments and related tax benefit	1,254	4	1,250
Stockholders' Equity, Ending Balance at Sep. 30, 2012	298,394	1,023	90,829	208,639	(2,097)
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2012		16,371
Stockholders' Equity, Beginning Balance at Dec. 31, 2012	295,652	1,024	91,453	213,674	(10,499)
Common Stock, Shares Outstanding, Beginning Balance at Dec. 31, 2012		16,380
Net income	26,364			26,364
Total other comprehensive income (loss), net of tax	9,159				9,159
Cash dividends declared	(12,686)			(12,686)
Share-based compensation	1,377		1,377
Deferred compensation plan, shares		2
Deferred compensation plan	30	0	30
Exercise of stock options, issuance of other compensation-related equity instruments and related tax benefit, shares		207
Exercise of stock options, issuance of other compensation-related equity instruments and related tax benefit	3,689	13	3,676
Stockholders' Equity, Ending Balance at Sep. 30, 2013	$ 323,585	$ 1,037	$ 96,536	$ 227,352	$ (1,340)
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2013		16,589