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	17,151			17,151
Total other comprehensive income (loss), net of tax	(141)				(141)
Cash dividends declared	(7,623)			(7,623)
Share-based compensation	876		876
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		57
Exercise of stock options, issuance of other compensation-related equity instruments and related tax benefit	974	3	971
Stockholders' Equity, Ending Balance at Jun. 30, 2012	292,734	1,022	90,022	203,726	(2,036)
Common Stock, Shares Outstanding, Ending Balance at Jun. 30, 2012		16,359
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	16,404			16,404
Total other comprehensive income (loss), net of tax	(2,196)				(2,196)
Cash dividends declared	(8,317)			(8,317)
Share-based compensation	879		879
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		105
Exercise of stock options, issuance of other compensation-related equity instruments and related tax benefit	918	6	912
Stockholders' Equity, Ending Balance at Jun. 30, 2013	$ 303,370	$ 1,030	$ 93,274	$ 221,761	$ (12,695)
Common Stock, Shares Outstanding, Ending Balance at Jun. 30, 2013		16,487