Consolidated Statements Of Changes In Shareholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Preferred stock	Common stock	Surplus	Treasury stock	Retained earnings	Accumulated other comprehensive loss	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjustment Retained earnings
Balance at Dec. 31, 2018	$ 3,267,570	$ 125,000	$ 56,518	$ 1,557,984	$ (5,634)	$ 1,610,574	$ (76,872)	$ (1,531)	$ (1,531)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	269,733					269,733
Other comprehensive income, net of tax	37,995						37,995
Cash dividends declared on common stock	(42,461)					(42,461)
Dividends on preferred stock	(6,150)					(6,150)
Stock-based compensation	8,278			8,278
Common stock issued for:
Exercise of stock options and warrants	3,613		112	4,225	(724)
Restricted stock awards	(441)		144	(144)	(441)
Employee stock purchase plan	2,131		32	2,099
Director compensation plan	1,588		19	1,569
Balance at Sep. 30, 2019	$ 3,540,325	125,000	56,825	1,574,011	(6,799)	1,830,165	(38,877)
Common stock issued for:
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201708Member
Balance at Jun. 30, 2019	$ 3,446,950	125,000	56,794	1,569,969	(6,650)	1,747,266	(45,429)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	99,121					99,121
Other comprehensive income, net of tax	6,552						6,552
Cash dividends declared on common stock	(14,172)					(14,172)
Dividends on preferred stock	(2,050)					(2,050)
Stock-based compensation	1,995			1,995
Common stock issued for:
Exercise of stock options and warrants	586		17	718	(149)
Restricted stock awards			2	(2)
Employee stock purchase plan	782		11	771
Director compensation plan	561		1	560
Balance at Sep. 30, 2019	3,540,325	125,000	56,825	1,574,011	(6,799)	1,830,165	(38,877)
Balance at Dec. 31, 2019	3,691,250	$ 125,000	57,951	1,650,278	(6,931)	1,899,630	(34,678)	$ (26,717)	$ (26,717)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	191,786					191,786
Other comprehensive income, net of tax	33,837						33,837
Cash dividends declared on common stock	(48,364)					(48,364)
Dividends on preferred stock	(14,386)					(14,386)
Common stock repurchased under authorized program	(37,116)				(37,116)
Stock-based compensation	(1,023)			(1,023)
Issuance of Series E preferred stock (in shares)		287,500
Issuance of Series E preferred stock	277,643			(9,857)
Common stock issued for:
Exercise of stock options and warrants	3,936		105	3,923	(92)
Restricted stock awards	(752)		195	(195)	(752)
Employee stock purchase plan	2,232		52	2,180
Director compensation plan	1,763		20	1,743
Balance at Sep. 30, 2020	$ 4,074,089	$ 412,500	58,323	1,647,049	(44,891)	2,001,949	(841)
Common stock issued for:
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Balance at Jun. 30, 2020	$ 3,990,218	412,500	58,294	1,643,864	(44,891)	1,921,048	(597)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	107,315					107,315
Other comprehensive income, net of tax	(244)						(244)
Cash dividends declared on common stock	(16,128)					(16,128)
Dividends on preferred stock	(10,286)					(10,286)
Stock-based compensation	1,255			1,255
Common stock issued for:
Exercise of stock options and warrants	497		7	490
Restricted stock awards			2	(2)
Employee stock purchase plan	847		20	827
Director compensation plan	615			615
Balance at Sep. 30, 2020	$ 4,074,089	$ 412,500	$ 58,323	$ 1,647,049	$ (44,891)	$ 2,001,949	$ (841)