Consolidated Statements of Shareholders' Equity (USD $) In Thousands, except Share data	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Deferred Compensation [Member]	Accumulated Other Comprehensive Earnings (Losses) [Member]
BEGINNING BALANCE at Dec. 31, 2009	$ 415,702	$ 208	$ 269,294	$ 115,123	$ (3,833)	$ 3,833	$ 31,077
BEGINNING BALANCE, shares at Dec. 31, 2009		20,826,431			(162,836)
Net earnings	59,659			59,659
Stock issued in acquisition of Sam Houston Financial Corp.	4,032	1	4,031
Stock issued in acquisition of Sam Houston Financial Corp., shares		85,306
Stock option exercises	789		789
Stock option exercises, shares		30,404
Cash dividends declared	(28,385)			(28,385)
Minimum liability pension adjustment, net of related income taxes	(423)						(423)
Change in unrealized gain on investment in securities available-for-sale, net of related income taxes	(10,201)						(10,201)
Additional tax benefit related to directors' deferred compensation plan	128		128
Shares purchased in connection with directors' deferred compensation plan, net					(374)	374
Shares purchased in connection with directors' deferred compensation plan, net, shares					(3,493)
Stock option expense	387		387
ENDING BALANCE at Dec. 31, 2010	441,688	209	274,629	146,397	(4,207)	4,207	20,453
ENDING BALANCE, shares at Dec. 31, 2010		20,942,141			(166,329)
Net earnings	68,369			68,369
Stock option exercises	950		950
Stock option exercises, shares		36,317
Cash dividends declared	(29,790)			(29,790)
Minimum liability pension adjustment, net of related income taxes	(1,690)						(1,690)
Change in unrealized gain on investment in securities available-for-sale, net of related income taxes	28,462						28,462
Additional tax benefit related to directors' deferred compensation plan	121		121
Shares purchased in connection with directors' deferred compensation plan, net					(390)	390
Shares purchased in connection with directors' deferred compensation plan, net, shares					(8,760)
Stock option expense	427		427
Three-for-two stock split in the form of a 50% stock dividend		105		(105)
Three-for-two stock split in the form of a 50% stock dividend, shares		10,481,177			(83,146)
ENDING BALANCE at Dec. 31, 2011	508,537	314	276,127	184,871	(4,597)	4,597	47,225
ENDING BALANCE, shares at Dec. 31, 2011		31,459,635			(258,235)
Net earnings	74,225			74,225
Stock option exercises	825	1	824
Stock option exercises, shares		37,246
Cash dividends declared	(31,169)			(31,169)
Minimum liability pension adjustment, net of related income taxes	(1,017)						(1,017)
Change in unrealized gain on investment in securities available-for-sale, net of related income taxes	5,101						5,101
Additional tax benefit related to directors' deferred compensation plan	127		127
Shares purchased in connection with directors' deferred compensation plan, net					(410)	410
Shares purchased in connection with directors' deferred compensation plan, net, shares					(8,610)
Stock option expense	334		334
ENDING BALANCE at Dec. 31, 2012	$ 556,963	$ 315	$ 277,412	$ 227,927	$ (5,007)	$ 5,007	$ 51,309
ENDING BALANCE, shares at Dec. 31, 2012		31,496,881			(266,845)