Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Treasury Stock [Member]
Total shareholders’ equity at beginning of period at Dec. 31, 2016	$ 3,002,528	$ 144,486	$ 637	$ 906,732	$ 1,985,569	$ (24,623)	$ (10,273)
Net income	263,625				263,625
Other comprehensive income (loss)	82,298					82,298
Stock option exercises/stock unit conversions (19,232 shares QTD and 447,831 shares YTD in 2018 and 22,724 shares QTD and 774,799 shares YTD in 2017)	45,422		5	36,148	(2,134)		11,403
Stock compensation expense recognized in earnings	9,013			9,013
Purchase of treasury stock (no shares QTD and 601 shares YTD in 2018 and 1,134,966 shares QTD and 1,135,435 shares YTD in 2017)	(100,042)						(100,042)
Cash dividends – preferred stock (approximately $0.34 per share QTD in both 2018 and in 2017 and $1.01 per share YTD in both 2018 and in 2017.)	(6,047)				(6,047)
Cash dividends – common stock ($0.67 per share QTD and $1.91 per share YTD in 2018 and $0.57 per share QTD and $1.68 per share YTD in 2017)	(107,754)				(107,754)
Total shareholders’ equity at end of period at Sep. 30, 2017	3,189,043	144,486	642	951,893	2,133,259	57,675	(98,912)
Total shareholders’ equity at beginning of period at Jun. 30, 2017	3,223,929	144,486	642	948,593	2,078,898	51,310	0
Net income	93,131				93,131
Other comprehensive income (loss)	6,365					6,365
Stock option exercises/stock unit conversions (19,232 shares QTD and 447,831 shares YTD in 2018 and 22,724 shares QTD and 774,799 shares YTD in 2017)	1,273			578	(393)		1,088
Stock compensation expense recognized in earnings	2,722			2,722
Purchase of treasury stock (no shares QTD and 601 shares YTD in 2018 and 1,134,966 shares QTD and 1,135,435 shares YTD in 2017)	(100,000)						(100,000)
Cash dividends – preferred stock (approximately $0.34 per share QTD in both 2018 and in 2017 and $1.01 per share YTD in both 2018 and in 2017.)	(2,016)				(2,016)
Cash dividends – common stock ($0.67 per share QTD and $1.91 per share YTD in 2018 and $0.57 per share QTD and $1.68 per share YTD in 2017)	(36,361)				(36,361)
Total shareholders’ equity at end of period at Sep. 30, 2017	3,189,043	144,486	642	951,893	2,133,259	57,675	(98,912)
Total shareholders' equity at beginning of period, as adjusted	3,295,578	144,486	642	953,361	2,184,784	79,512	(67,207)
Total shareholders’ equity at beginning of period at Dec. 31, 2017	3,297,863	144,486	642	953,361	2,187,069	79,512	(67,207)
Cumulative effect of accounting change at Dec. 31, 2017	(2,285)				(2,285)
Net income	335,665				335,665
Other comprehensive income (loss)	(230,282)					(230,282)
Reclassification of certain income tax effects related to U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act	9,535				(9,535)	9,535
Stock option exercises/stock unit conversions (19,232 shares QTD and 447,831 shares YTD in 2018 and 22,724 shares QTD and 774,799 shares YTD in 2017)	26,567				(12,887)		39,454
Stock compensation expense recognized in earnings	9,556			9,556
Purchase of treasury stock (no shares QTD and 601 shares YTD in 2018 and 1,134,966 shares QTD and 1,135,435 shares YTD in 2017)	(70)						(70)
Cash dividends – preferred stock (approximately $0.34 per share QTD in both 2018 and in 2017 and $1.01 per share YTD in both 2018 and in 2017.)	(6,047)				(6,047)
Cash dividends – common stock ($0.67 per share QTD and $1.91 per share YTD in 2018 and $0.57 per share QTD and $1.68 per share YTD in 2017)	(122,721)				(122,721)
Total shareholders’ equity at end of period at Sep. 30, 2018	3,308,246	144,486	642	962,917	2,369,259	(141,235)	(27,823)
Total shareholders’ equity at beginning of period at Jun. 30, 2018	3,309,512	144,486	642	960,121	2,297,099	(63,319)	(29,517)
Net income	117,844				117,844
Other comprehensive income (loss)	(77,916)					(77,916)
Stock option exercises/stock unit conversions (19,232 shares QTD and 447,831 shares YTD in 2018 and 22,724 shares QTD and 774,799 shares YTD in 2017)	1,119				(575)		1,694
Stock compensation expense recognized in earnings	2,796			2,796
Cash dividends – preferred stock (approximately $0.34 per share QTD in both 2018 and in 2017 and $1.01 per share YTD in both 2018 and in 2017.)	(2,016)				(2,016)
Cash dividends – common stock ($0.67 per share QTD and $1.91 per share YTD in 2018 and $0.57 per share QTD and $1.68 per share YTD in 2017)	(43,093)				(43,093)
Total shareholders’ equity at end of period at Sep. 30, 2018	$ 3,308,246	$ 144,486	$ 642	$ 962,917	$ 2,369,259	$ (141,235)	$ (27,823)