Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Series D Preferred Stock	Series E Preferred Stock	Preferred stock	Preferred stock Series E Preferred Stock	Common stock	Surplus	Surplus Series E Preferred Stock	Treasury stock	Retained earnings	Accumulated other comprehensive (loss) income	Cumulative effect, period of adoption, adjustment	Cumulative effect, period of adoption, adjustment Retained earnings
Beginning 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 loss, 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 repurchases under authorized program	(37,116)								(37,116)
Stock-based compensation	(1,023)						(1,023)
Issuance of Series E preferred stock			$ 277,643					$ (9,857)
Issuance of Series E preferred stock (in shares)					287,500
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
Ending 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:
Dividends on preferred stock (usd per share)		$ 1.23	$ 716.15
Beginning 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 loss, 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	0					2	(2)
Employee stock purchase plan	847					20	827
Director compensation plan	615						615
Ending 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:
Dividends on preferred stock (usd per share)		0.41	716.15
Beginning balance at Dec. 31, 2020	4,115,995			412,500		58,473	1,649,990		(100,363)	2,080,013	15,382
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	367,394									367,394
Other comprehensive loss, net of tax	(13,965)										(13,965)
Cash dividends declared on common stock	(52,987)									(52,987)
Dividends on preferred stock	(20,973)									(20,973)
Common stock repurchases under authorized program	(9,540)								(9,540)
Stock-based compensation	9,437						9,437
Common stock issued for:
Exercise of stock options and warrants	10,559					246	10,313
Restricted stock awards	0					13	(13)
Employee stock purchase plan	2,548					38	2,510
Director compensation plan	1,849					24	1,825
Ending balance at Sep. 30, 2021	4,410,317			412,500		58,794	1,674,062		(109,903)	2,373,447	1,417
Common stock issued for:
Dividends on preferred stock (usd per share)		1.23	1,289.07
Beginning balance at Jun. 30, 2021	4,339,011			412,500		58,770	1,669,002		(100,363)	2,288,969	10,133
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	109,137									109,137
Other comprehensive loss, net of tax	(8,716)										(8,716)
Cash dividends declared on common stock	(17,668)									(17,668)
Dividends on preferred stock	(6,991)									(6,991)
Stock-based compensation	3,243						3,243
Common stock issued for:
Exercise of stock options and warrants	318					8	310
Restricted stock awards	0					3	(3)
Employee stock purchase plan	928					13	915
Director compensation plan	595						595
Ending balance at Sep. 30, 2021	$ 4,410,317			$ 412,500		$ 58,794	$ 1,674,062		$ (109,903)	$ 2,373,447	$ 1,417
Common stock issued for:
Dividends on preferred stock (usd per share)		$ 0.41	$ 429.69