Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands		Total	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, 2016		$ 1,151,111	$ 626	$ 711,663	$ (15,212)	$ 454,034
Comprehensive income		78,598			4,561	74,037
Stock issued for employee stock purchase plan		618		618
Stock-based compensation plans, net		8,220	2	8,218
Stock issued for acquisition							$ 42,638	$ 16	$ 42,622
Dividends on common stock	[1]	(23,986)				(23,986)
Balance at Sep. 30, 2017		1,257,199	644	763,121	(10,651)	504,085
Comprehensive income		15,306			(3,597)	18,903
Reclassify stranded tax effects due to 2017 tax law changes		0			(3,016)	3,016
Stock-based compensation plans, net		4,721	1	4,720
Stock issued for acquisition										$ 431,398	$ 145	$ 431,253	$ 387,070	$ 130	$ 386,940
Dividends on common stock	[1]	(11,130)				(11,130)
Balance at Dec. 31, 2017		2,084,564	920	1,586,034	(17,264)	514,874
Comprehensive income		129,291			(30,776)	160,067
Stock issued for employee stock purchase plan		1,026		1,026
Stock-based compensation plans, net		10,204	3	10,201
Dividends on common stock	[1]	(41,766)				(41,766)
Balance at Sep. 30, 2018		$ 2,183,319	$ 923	$ 1,597,261	$ (48,040)	$ 633,175

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