Consolidated Statements Of Changes In Shareholders' Equity - USD ($) $ in Thousands	Total	Preferred stock	Common stock	Surplus	Treasury stock	Retained earnings	Accumulated other comprehensive income (loss)	Series C preferred stock	Series C preferred stock Preferred stock	Series C preferred stock Common stock	Series C preferred stock Surplus
Balance at beginning of period at Dec. 31, 2015	$ 2,352,274	$ 251,287	$ 48,469	$ 1,190,988	$ (3,973)	$ 928,211	$ (62,708)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	206,875					206,875
Other comprehensive income (loss), net of tax	(2,620)						(2,620)
Cash dividends declared on common stock	(24,055)					(24,055)
Dividends on preferred stock	(14,513)					(14,513)
Stock-based compensation	9,303			9,303
Conversion of Series C Preferred Stock to common stock								$ 0	$ (30)	$ 1	$ 29
Common stock issued for:
New issuance, net of costs	152,911		3,000	149,911
Exercise of stock options and warrants	11,228		329	11,276	(377)
Restricted stock awards	1		98	142	(239)
Employee stock purchase plan	2,637		56	2,581
Director compensation plan	1,576		25	1,551
Balance at end of period at Dec. 31, 2016	2,695,617	251,257	51,978	1,365,781	(4,589)	1,096,518	(65,328)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	257,682					257,682
Other comprehensive income (loss), net of tax	23,493						23,493
Cash dividends declared on common stock	(30,765)					(30,765)
Dividends on preferred stock	(9,778)					(9,778)
Stock-based compensation	12,858			12,858
Conversion of Series C Preferred Stock to common stock								$ 0	$ (126,257)	$ 3,121	$ 123,136
Common stock issued for:
Exercise of stock options and warrants	24,074		813	23,261	0
Restricted stock awards	(397)		88	(88)	(397)
Employee stock purchase plan	2,530		36	2,494
Director compensation plan	1,625		32	1,593
Balance at end of period at Dec. 31, 2017	2,976,939	125,000	56,068	1,529,035	(4,986)	1,313,657	(41,835)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment of stranded tax effects - ASU 2018-02	(2,974)					2,974	(2,974)
Balance at beginning of period at Dec. 31, 2017	2,976,939	125,000	56,068	1,529,035	(4,986)	1,313,657	(41,835)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	343,166					343,166
Other comprehensive income (loss), net of tax	(30,183)						(30,183)
Cash dividends declared on common stock	(42,787)					(42,787)
Dividends on preferred stock	(8,200)					(8,200)
Stock-based compensation	13,496			13,496
Common stock issued for:
Exercise of stock options and warrants	11,466		299	11,359	(192)
Restricted stock awards	(456)		101	(101)	(456)
Employee stock purchase plan	2,523		31	2,492
Director compensation plan	1,722		19	1,703
Balance at end of period at Dec. 31, 2018	3,267,570	$ 125,000	$ 56,518	$ 1,557,984	$ (5,634)	1,610,574	(76,872)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment from the adoption of ASUs | ASU 2016-01	(1,880)					1,880	$ (1,880)
Cumulative effect adjustment from the adoption of ASUs | ASU 2017-12	$ (116)					$ (116)