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, 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 dividends ($0.20 per share in 2015 and $0.22 per share in 2016)	(30,508)	0	0	0	(30,508)	0	0
Preferred stock dividends	(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
Stockholders' Equity, Other	(706)	0	0	0	(706)	0	0
Stock-based compensation expense, net	10,879	0	0	10,879	0	0	0
Tax impact 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)
Beginning balance at Dec. 31, 2015	2,937,246	121,379	1,642	1,458,522	1,593,239	(32,616)	(204,920)
Comprehensive income:
Net income	91,625	0	0	0	91,625	0	0
Other comprehensive income	46,069	0	0	0	0	46,069	0
Comprehensive income	137,694
Common stock issued:
Stock-based compensation plans, net	4,276	0	0	1,153	(17,478)	0	20,601
Purchase of common stock returned to authorized but unissued	(20,007)	0	(12)	(19,995)	0	0	0
Purchase of treasury stock	(4,018)	0	0	0	0	0	(4,018)
Cash dividends:
Common stock dividends ($0.20 per share in 2015 and $0.22 per share in 2016)	(33,034)	0	0	0	(33,034)	0	0
Preferred stock dividends	(4,367)	0	0	0	(4,367)	0	0
Issuance of preferred stock	0
Purchase of preferred stock	(1,248)	(1,178)	0	0	(70)	0	0
Stock-based compensation expense, net	13,210	0	0	13,210	0	0	0
Tax impact of stock-based compensation	395	0	0	395	0	0	0
Ending balance at Jun. 30, 2016	$ 3,030,147	$ 120,201	$ 1,630	$ 1,453,285	$ 1,629,915	$ 13,453	$ (188,337)