Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands		Total	Preferred Stock	Common Stock	Surplus	Accumulated Other Comprehensive Income (Loss)	Undivided Profits	Hardeman County Investment Company, Inc.	Hardeman County Investment Company, Inc. Common Stock	Hardeman County Investment Company, Inc. Surplus	OKSB Merger	OKSB Merger Common Stock	OKSB Merger Surplus	First Texas Merger	First Texas Merger Common Stock	First Texas Merger Surplus
Balance at Dec. 31, 2015		$ 1,076,855	$ 30,852	$ 606	$ 662,075	$ (2,665)	$ 385,987
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income		84,267				(12,547)	96,814
Stock issued for employee stock purchase plan		586			586
Stock-based compensation plans, net		7,770		3	7,767
Stock issued for acquisition		41,252		17	41,235
Preferred stock redeemed		(30,852)	(30,852)
Dividends on preferred stock		(24)					(24)
Dividends on common stock	[1]	(28,743)					(28,743)
Balance at Dec. 31, 2016		1,151,111	0	626	711,663	(15,212)	454,034
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income		93,904				964	92,940
Reclassify stranded tax effects due to 2017 tax law changes		0				(3,016)	3,016
Stock issued for employee stock purchase plan		618			618
Stock-based compensation plans, net		12,941		3	12,938
Stock issued for acquisition								$ 42,638	$ 16	$ 42,622	$ 431,398	$ 145	$ 431,253	$ 387,070	$ 130	$ 386,940
Dividends on common stock	[1]	(35,116)					(35,116)
Balance at Dec. 31, 2017		2,084,564	0	920	1,586,034	(17,264)	514,874
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income		205,603				(10,110)	215,713
Stock issued for employee stock purchase plan		1,026			1,026
Stock-based compensation plans, net		10,887		3	10,884
Dividends on common stock		(55,646)					(55,646)
Balance at Dec. 31, 2018		$ 2,246,434	$ 0	$ 923	$ 1,597,944	$ (27,374)	$ 674,941

[1] All share and per share amounts for the years ended 2017 and 2016 have been restated to reflect the effect of the two-for-one stock split on February 8, 2018.