Consolidated Statements Of Changes In Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Preferred stock	Common stock	Surplus	Treasury stock	Retained earnings	Accumulated other comprehensive loss	Series C Preferred Stock Preferred stock	Series C Preferred Stock Common stock	Series C Preferred Stock Surplus
Balance at the beginning 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	188,901					188,901
Other comprehensive income, net of tax	23,842						23,842
Cash dividends declared on common stock	(22,932)					(22,932)
Dividends on preferred stock	(7,728)					(7,728)
Stock-based compensation	8,160			8,160
Conversion of Series C preferred stock to common stock								$ (126,257)	$ 3,121	$ 123,136
Common stock issued for:
Exercise of stock options and warrants	20,246		702	19,544
Restricted stock awards	(295)		79	(79)	(295)
Employee stock purchase plan	1,921		28	1,893
Director compensation plan	1,193		32	1,161
Balance at the end at Sep. 30, 2017	2,908,925	125,000	55,940	1,519,596	(4,884)	1,254,759	(41,486)
Balance at the beginning at Jun. 30, 2017							(46,537)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	65,626
Other comprehensive income, net of tax	5,051
Balance at the end at Sep. 30, 2017	2,908,925	125,000	55,940	1,519,596	(4,884)	1,254,759	(41,486)
Balance at the beginning 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	263,509					263,509
Other comprehensive income, net of tax	(46,461)						(46,461)
Cash dividends declared on common stock	(32,074)					(32,074)
Dividends on preferred stock	(6,150)					(6,150)
Stock-based compensation	10,346			10,346
Common stock issued for:
Exercise of stock options and warrants	11,043		283	10,952	(192)
Restricted stock awards	(369)		93	(93)	(369)
Employee stock purchase plan	1,885		23	1,862
Director compensation plan	1,270		19	1,251
Balance at the end at Sep. 30, 2018	3,179,822	125,000	56,486	1,553,353	(5,547)	1,543,680	(93,150)
Balance at the beginning at Jun. 30, 2018							(81,216)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	91,948
Other comprehensive income, net of tax	(11,934)
Balance at the end at Sep. 30, 2018	$ 3,179,822	$ 125,000	$ 56,486	$ 1,553,353	$ (5,547)	$ 1,543,680	$ (93,150)