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	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 – 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	(30,265)					(30,265)
Balance at Jun. 30, 2019	2,469,513	0	966	1,705,262	15,316	747,969
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 Mar. 31, 2019	$ 2,302,321	0	926	1,599,566	(6,000)	707,829
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	77,240				21,316	55,924
Stock-based compensation plans, net	2,906			2,906
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	(15,458)					(15,458)
Balance at Jun. 30, 2019	2,469,513	0	966	1,705,262	15,316	747,969
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	169,457				33,419	136,038
Stock issued for employee stock purchase plan	956		1	955
Stock-based compensation plans, net	4,406		2	4,404
Stock repurchases – 4,922,336 shares	(93,307)		(49)	(93,258)
Dividends on preferred stock	(26)					(26)
Dividends on common stock	(37,606)					(37,606)
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]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Balance at Mar. 31, 2020	$ 2,845,400	767	1,090	2,026,420	38,230	778,893
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	74,882				16,080	58,802
Stock-based compensation plans, net	2,963			2,963
Dividends on preferred stock	(13)					(13)
Dividends on common stock	(18,529)					(18,529)
Balance at Jun. 30, 2020	$ 2,904,703	$ 767	$ 1,090	$ 2,029,383	$ 54,310	$ 819,153