Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Surplus	Accumulated Other Comprehensive Income (Loss)	Undivided Profits	Impact of adoption of 2016-13	Impact of adoption of 2016-13 Undivided Profits
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	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	0	926	1,599,566	(6,000)	707,829
Balance at Dec. 31, 2019	2,988,924	767	1,136	2,117,282	20,891	848,848	$ (128,101)	$ (128,101)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	94,575				17,339	77,236
Stock issued for employee stock purchase plan	956		1	955
Stock-based compensation plans, net	1,443		2	1,441
Stock repurchases – 4,922,336 shares	(93,307)		(49)	(93,258)
Dividends on preferred stock	(13)					(13)
Dividends on common stock	(19,077)					(19,077)
Balance at Mar. 31, 2020	$ 2,845,400	$ 767	$ 1,090	$ 2,026,420	$ 38,230	$ 778,893