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	Reliance Bancshares, Inc.	Reliance Bancshares, Inc. Preferred Stock	Reliance Bancshares, Inc. Common Stock	Reliance Bancshares, Inc. Surplus	Landrum Company	Landrum Company Preferred Stock	Landrum Company Common Stock	Landrum Company Surplus
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	(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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	286,432				48,265	238,167
Stock issued for employee stock purchase plan	1,312		1	1,311
Stock-based compensation plans, net	10,532		3	10,529
Stock issued for acquisition																$ 144,830	$ 42,000	$ 40	$ 102,790	$ 415,772	$ 767	$ 173	$ 414,832
Preferred stock retirement	(42,000)	(42,000)
Stock repurchases - 390,000 shares	(10,128)		(4)	(10,124)
Dividends on preferred stock	(339)					(339)
Dividends on common stock	(63,921)					(63,921)
Balance at Dec. 31, 2019	$ 2,988,924	$ 767	$ 1,136	$ 2,117,282	$ 20,891	$ 848,848