Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	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	34,514			(16,798)	51,312
Stock issued for employee stock purchase plan	1,026		1,026
Stock-based compensation plans, net	3,028	2	3,026
Dividends on common stock	(14,081)				(14,081)
Balance at Mar. 31, 2018	2,109,051	922	1,590,086	(34,062)	552,105
Comprehensive income	171,089			6,688	164,401
Stock-based compensation plans, net	7,859	1	7,858
Dividends on common stock	(41,565)				(41,565)
Balance at Dec. 31, 2018	2,246,434	923	1,597,944	(27,374)	674,941
Comprehensive income	69,069			21,374	47,695
Stock issued for employee stock purchase plan	1,312	1	1,311
Stock-based compensation plans, net	313	2	311
Dividends on common stock	(14,807)				(14,807)
Balance at Mar. 31, 2019	$ 2,302,321	$ 926	$ 1,599,566	$ (6,000)	$ 707,829