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	81,955				(22,919)	104,874
Stock issued for employee stock purchase plan	1,026			1,026
Stock-based compensation plans, net	7,285		3	7,282
Dividends on common stock	(27,922)					(27,922)
Balance at Jun. 30, 2018	2,146,908		923	1,594,342	(40,183)	591,826
Balance at Mar. 31, 2018	2,109,051		922	1,590,086	(34,062)	552,105
Comprehensive income	47,441				(6,121)	53,562
Stock-based compensation plans, net	4,257		1	4,256
Dividends on common stock	(13,841)					(13,841)
Balance at Jun. 30, 2018	2,146,908		923	1,594,342	(40,183)	591,826
Balance at Dec. 31, 2018	2,246,434		923	1,597,944	(27,374)	674,941
Comprehensive income	146,309				42,690	103,619
Stock issued for employee stock purchase plan	1,312		1	1,311
Stock-based compensation plans, net	3,219		2	3,217
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	(30,265)					(30,265)
Balance at Jun. 30, 2019	2,469,513		966	1,705,262	15,316	747,969
Balance at Mar. 31, 2019	2,302,321		926	1,599,566	(6,000)	707,829
Comprehensive income	77,240				21,316	55,924
Stock-based compensation plans, net	2,906			2,906
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	(15,458)					(15,458)
Balance at Jun. 30, 2019	$ 2,469,513		$ 966	$ 1,705,262	$ 15,316	$ 747,969