Consolidated Statement of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), Net of Tax [Member]	Treasury Stock [Member]
Balance at Dec. 31, 2014	$ 2,851,403	$ 144,486	$ 637	$ 886,476	$ 1,710,324	$ 141,814	$ (32,334)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	279,328				279,328
Other comprehensive loss, net of tax	(27,951)					(27,951)
Proceeds from stock option exercises/stock unit conversions	14,853			(2,248)	(4,240)		21,341
Tax benefits from stock-based compensation	1,434			1,434
Stock-based compensation expense recognized in earnings	12,737			12,737
Non-vested stock awards and stock units	0			(1,049)			1,049
Purchase of treasury stock	(101,237)						(101,237)
Cash dividends paid on preferred stock	(8,063)				(8,063)
Cash dividends – common stock	(132,161)				(132,161)
Balance at Dec. 31, 2015	2,890,343	144,486	637	897,350	1,845,188	113,863	(111,181)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	304,261				304,261
Other comprehensive loss, net of tax	(138,486)					(138,486)
Proceeds from stock option exercises/stock unit conversions	78,866			(2,417)	(20,915)		102,198
Stock-based compensation expense recognized in earnings	11,799			11,799
Purchase of treasury stock	(1,290)						(1,290)
Cash dividends paid on preferred stock	(8,063)				(8,063)
Cash dividends – common stock	(134,902)				(134,902)
Balance at Dec. 31, 2016	3,002,528	144,486	637	906,732	1,985,569	(24,623)	(10,273)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	364,149				364,149
Other comprehensive loss, net of tax	104,135					104,135
Proceeds from stock option exercises/stock unit conversions	67,746		5	33,616	(10,414)		44,539
Stock-based compensation expense recognized in earnings	13,013			13,013
Purchase of treasury stock	(101,473)						(101,473)
Cash dividends paid on preferred stock	(8,063)				(8,063)
Cash dividends – common stock	(144,172)				(144,172)
Balance at Dec. 31, 2017	$ 3,297,863	$ 144,486	$ 642	$ 953,361	$ 2,187,069	$ 79,512	$ (67,207)