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	46,672	0	0	0	46,672	0	0
Other comprehensive income	29,650	0	0	0	0	29,650	0
Comprehensive income	76,322
Common stock issued:
Stock-based compensation plans, net	6,236	0	0	304	(18,015)	0	23,947
Acquisition of Ahmann & Martin Co.	43,530	0	26	43,504	0	0	0
Purchase of treasury stock	(4,105)	0	0	0	0	0	(4,105)
Cash dividends:
Common stock dividends ($0.10 per share in 2015 and $0.11 per share in 2016)	(15,280)	0	0	0	(15,280)	0	0
Preferred stock dividends	(1,228)	0	0	0	(1,228)	0	0
Stock-based compensation expense, net	5,774	0	0	5,774	0	0	0
Tax impact of stock-based compensation	638	0	0	638	0	0	0
Purchase of common stock returned to authorized but unissued	(30,000)	0	(17)	(29,983)	0	0	0
Ending balance at Mar. 31, 2015	2,882,138	59,727	1,674	1,505,170	1,509,967	24,800	(219,200)
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	42,534	0	0	0	42,534	0	0
Other comprehensive income	34,783	0	0	0	0	34,783	0
Comprehensive income	77,317
Common stock issued:
Stock-based compensation plans, net	2,205	0	0	613	(17,271)	0	18,863
Purchase of treasury stock	(2,792)	0	0	0	0	0	(2,792)
Cash dividends:
Common stock dividends ($0.10 per share in 2015 and $0.11 per share in 2016)	(16,409)	0	0	0	(16,409)	0	0
Preferred stock dividends	(2,198)	0	0	0	(2,198)	0	0
Purchase of preferred stock	(1,092)	(1,032)	0	0	(60)	0	0
Stock-based compensation expense, net	8,066	0	0	8,066	0	0	0
Tax impact of stock-based compensation	162	0	0	162	0	0	0
Purchase of common stock returned to authorized but unissued	(20,007)	0	(12)	(19,995)	0	0	0
Ending balance at Mar. 31, 2016	$ 2,982,498	$ 120,347	$ 1,630	$ 1,447,368	$ 1,599,835	$ 2,167	$ (188,849)