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	91,564	0	0	0	91,564	0	0
Other comprehensive income	34,738	0	0	0	0	34,738	0
Comprehensive income	126,302
Common stock issued:
Stock-based compensation plans, net	8,363	0	0	1,071	(19,735)	0	27,027
Purchase of treasury stock	(72,647)	0	0	0	0	0	(72,647)
Cash dividends:
Common stock ($0.18 per share in 2014 and $0.20 per share in 2015)	(29,175)	0	0	0	(29,175)	0	0
Preferred stock	(2,522)	0	0	0	(2,522)	0	0
Issuance of preferred stock	0
Purchase of preferred stock	(960)	(838)	0	0	(122)	0	0
Stock-based compensation expense, net	8,468	0	0	8,468	0	0	0
Tax benefit of stock-based compensation	827	0	0	827	0	0	0
Ending balance at Jun. 30, 2014	2,929,946	61,024	1,750	1,628,356	1,432,518	10,494	(204,196)
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	96,072	0	0	0	96,072	0	0
Other comprehensive income	7,444	0	0	0	0	7,444	0
Comprehensive income	103,516
Common stock issued:
Stock-based compensation plans, net	13,704	0	0	2,051	(21,145)	0	32,798
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	(4,500)	0	0	0	0	0	(4,500)
Cash dividends:
Common stock ($0.18 per share in 2014 and $0.20 per share in 2015)	(30,508)	0	0	0	(30,508)	0	0
Preferred stock	(2,773)	0	0	0	(2,773)	0	0
Issuance of preferred stock	62,966	62,966	0	0	0	0	0
Purchase of preferred stock	(752)	(678)	0	0	(74)	0	0
Other	706	0	0	0	706	0	0
Stock-based compensation expense, net	10,879	0	0	10,879	0	0	0
Tax benefit of stock-based compensation	1,784	0	0	1,784	0	0	0
Ending balance at Jun. 30, 2015	$ 2,904,391	$ 122,015	$ 1,642	$ 1,450,200	$ 1,538,684	$ 2,594	$ (210,744)