Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Surplus	Accumulated Other Comprehensive Income (Loss)	Undivided Profits	Impact of ASU 2016-13 adoption	Impact of ASU 2016-13 adoption 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	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 – 3,999,623 shares	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	0	966	1,708,058	23,709	814,338
Balance at Dec. 31, 2018	2,246,434	0	923	1,597,944	(27,374)	674,941
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]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Balance at Jun. 30, 2019	$ 2,469,513	0	966	1,705,262	15,316	747,969
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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	0	966	1,708,058	23,709	814,338
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	222,554				20,618	201,936
Stock issued for employee stock purchase plan	956		1	955
Stock-based compensation plans, net	7,395		2	7,393
Stock repurchases – 4,922,336 shares	(93,307)		(49)	(93,258)
Dividends on preferred stock	(39)					(39)
Dividends on common stock	(56,141)					(56,141)
Balance at Sep. 30, 2020	$ 2,942,241	767	1,090	2,032,372	41,509	866,503
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Balance at Jun. 30, 2020	$ 2,904,703	767	1,090	2,029,383	54,310	819,153
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	53,097				(12,801)	65,898
Stock-based compensation plans, net	2,989			2,989
Dividends on preferred stock	(13)					(13)
Dividends on common stock	(18,535)					(18,535)
Balance at Sep. 30, 2020	$ 2,942,241	$ 767	$ 1,090	$ 2,032,372	$ 41,509	$ 866,503