Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Surplus	Accumulated Other Comprehensive Income (Loss)	Undivided Profits
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	(41,766)					(41,766)
Balance at Sep. 30, 2018	2,183,319		923	1,597,261	(48,040)	633,175
Balance at Jun. 30, 2018	2,146,908		923	1,594,342	(40,183)	591,826
Comprehensive income	47,336				(7,857)	55,193
Stock-based compensation plans, net	2,919		0	2,919
Dividends on common stock	(13,844)					(13,844)
Balance at Sep. 30, 2018	2,183,319		923	1,597,261	(48,040)	633,175
Balance at Dec. 31, 2018	2,246,434		923	1,597,944	(27,374)	674,941
Comprehensive income	236,528				51,083	185,445
Stock issued for employee stock purchase plan	1,312		1	1,311
Stock-based compensation plans, net	6,015		2	6,013
Stock issued for Reliance acquisition	144,830	$ 42,000	40	102,790
Preferred stock retirement	(42,000)	$ (42,000)
Dividends on preferred stock	(326)					(326)
Dividends on common stock	(45,722)					(45,722)
Balance at Sep. 30, 2019	2,547,071		966	1,708,058	23,709	814,338
Balance at Jun. 30, 2019	2,469,513		966	1,705,262	15,316	747,969
Comprehensive income	90,219				8,393	81,826
Stock-based compensation plans, net	2,796			2,796
Dividends on common stock	(15,457)					(15,457)
Balance at Sep. 30, 2019	$ 2,547,071		$ 966	$ 1,708,058	$ 23,709	$ 814,338