Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Preferred Equity	Common Stock	Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
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 dividends ($0.30 per share in 2015 and $0.33 per share in 2016)	(45,599)	0	0	0	(45,599)	0	0
Preferred stock dividends	(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
Stockholders' Equity, 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)
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	145,441	0	0	0	145,441	0	0
Other comprehensive income	31,362	0	0	0	0	31,362	0
Comprehensive income	176,803
Common stock issued:
Stock-based compensation plans, net	7,749	0	0	1,785	(18,070)	0	24,034
Purchase of common stock returned to authorized but unissued	(20,007)	0	(12)	(19,995)	0	0	0
Purchase of treasury stock	(4,243)	0	0	0	0	0	(4,243)
Cash dividends:
Common stock dividends ($0.30 per share in 2015 and $0.33 per share in 2016)	(49,642)	0	0	0	(49,642)	0	0
Preferred stock dividends	(6,555)	0	0	0	(6,555)	0	0
Issuance of preferred stock	97,066	97,066	0	0	0	0	0
Redemption of preferred stock	(58,903)	(57,338)	0	0	(1,565)	0	0
Purchase of preferred stock	(1,248)	(1,178)	0	0	(70)	0	0
Stock-based compensation expense, net	18,047	0	0	18,047	0	0	0
Tax benefit of stock-based compensation	802	0	0	802	0	0	0
Ending balance at Sep. 30, 2016	$ 3,097,115	$ 159,929	$ 1,630	$ 1,459,161	$ 1,662,778	$ (1,254)	$ (185,129)