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	170,612					170,612
Other comprehensive income, net of tax	31,443						31,443
Cash dividends declared on common stock	(28,289)					(28,289)
Dividends on preferred stock	(4,100)					(4,100)
Stock-based compensation	6,283			6,283
Common stock issued for:
Exercise of stock options and warrants	3,027		95	3,507	(575)
Restricted stock awards	(441)		142	(142)	(441)
Employee stock purchase plan	1,349		21	1,328
Director compensation plan	1,027		18	1,009
Balance at Jun. 30, 2019	$ 3,446,950	125,000	56,794	1,569,969	(6,650)	1,747,266	(45,429)
Common stock issued for:
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201708Member
Balance at Mar. 31, 2019	$ 3,371,972	125,000	56,765	1,565,185	(6,650)	1,682,016	(50,344)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	81,466					81,466
Other comprehensive income, net of tax	4,915						4,915
Cash dividends declared on common stock	(14,166)					(14,166)
Dividends on preferred stock	(2,050)					(2,050)
Stock-based compensation	2,965			2,965
Common stock issued for:
Exercise of stock options and warrants	659		16	643
Restricted stock awards			3	(3)
Employee stock purchase plan	666		10	656
Director compensation plan	523			523
Balance at Jun. 30, 2019	3,446,950	125,000	56,794	1,569,969	(6,650)	1,747,266	(45,429)
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	84,471					84,471
Other comprehensive income, net of tax	34,081						34,081
Cash dividends declared on common stock	(32,236)					(32,236)
Dividends on preferred stock	(4,100)					(4,100)
Common stock repurchases	(37,116)				(37,116)
Stock-based compensation	(2,278)			(2,278)
Issuance of Series E preferred stock (in shares)		287,500
Issuance of Series E preferred stock	277,375			(10,125)
Common stock issued for:
Exercise of stock options and warrants	3,707		98	3,701	(92)
Restricted stock awards	(752)		193	(193)	(752)
Employee stock purchase plan	1,385		32	1,353
Director compensation plan	1,148		20	1,128
Balance at Jun. 30, 2020	$ 3,990,218	$ 412,500	58,294	1,643,864	(44,891)	1,921,048	(597)
Common stock issued for:
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Balance at Mar. 31, 2020	$ 3,700,393	$ 125,000	58,266	1,652,063	(44,891)	1,917,558	(7,603)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	21,659					21,659
Other comprehensive income, net of tax	7,006						7,006
Cash dividends declared on common stock	(16,119)					(16,119)
Dividends on preferred stock	(2,050)					(2,050)
Stock-based compensation	541			541
Issuance of Series E preferred stock (in shares)		287,500
Issuance of Series E preferred stock	277,375			(10,125)
Common stock issued for:
Exercise of stock options and warrants	161		3	158
Restricted stock awards			3	(3)
Employee stock purchase plan	676		22	654
Director compensation plan	576			576
Balance at Jun. 30, 2020	$ 3,990,218	$ 412,500	$ 58,294	$ 1,643,864	$ (44,891)	$ 1,921,048	$ (597)