Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Preferred Equity [Member]	Common Stock [Member]	Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]
Beginning balance at Dec. 31, 2013	$ 2,891,290	$ 61,862	$ 1,750	$ 1,617,990	$ 1,392,508	$ (24,244)	$ (158,576)
Comprehensive income:
Net income	141,771	0	0	0	141,771	0	0
Other comprehensive income	22,519	0	0	0	0	22,519	0
Comprehensive income	164,290
Common stock issued:
Stock-based compensation plans, net	11,131	0	0	1,863	(20,556)	0	29,824
Purchase of common stock returned to authorized but unissued	(50,498)	0	(31)	(50,467)	0	0	0
Purchase of treasury stock	(112,245)	0	0	0	0	0	(112,245)
Cash dividends:
Common stock ($0.27 per share in 2014 and $0.30 per share in 2015)	(43,299)	0	0	0	(43,299)	0	0
Preferred stock	(3,777)	0	0	0	(3,777)	0	0
Purchase of preferred stock	(960)	(838)	0	0	(122)	0	0
Stock-based compensation expense, net	12,462	0	0	12,462	0	0	0
Tax benefit of stock-based compensation	1,184	0	0	1,184	0	0	0
Ending balance at Sep. 30, 2014	2,869,578	61,024	1,719	1,583,032	1,466,525	(1,725)	(240,997)
Beginning balance at Dec. 31, 2014	2,800,251	59,727	1,665	1,484,933	1,497,818	(4,850)	(239,042)
Comprehensive income:
Net income	145,510	0	0	0	145,510	0	0
Other comprehensive income	20,226	0	0	0	0	20,226	0
Comprehensive income	165,736
Common stock issued:
Stock-based compensation plans, net	15,373	0	0	2,880	(21,786)	0	34,279
Acquisition of Ahmann & Martin Co.	43,530	0	26	43,504	0	0	0
Purchase of common stock returned to authorized but unissued	(93,000)	0	(49)	(92,951)	0	0	0
Purchase of treasury stock	(5,070)	0	0	0	0	0	(5,070)
Cash dividends:
Common stock ($0.27 per share in 2014 and $0.30 per share in 2015)	(45,599)	0	0	0	(45,599)	0	0
Preferred stock	(4,957)	0	0	0	(4,957)	0	0
Issuance of preferred stock	62,966	62,966	0	0	0	0	0
Purchase of preferred stock	(1,335)	(1,209)	0	0	(126)	0	0
Other	(766)	(105)	0	0	(661)	0	0
Stock-based compensation expense, net	14,575	0	0	14,575	0	0	0
Tax benefit of stock-based compensation	2,093	0	0	2,093	0	0	0
Ending balance at Sep. 30, 2015	$ 2,953,797	$ 121,379	$ 1,642	$ 1,455,034	$ 1,570,199	$ 15,376	$ (209,833)