Consolidated Statements of Changes in Stockholders' Equity (Unaudited) (USD $) In Thousands, unless otherwise specified	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	45,199	0	0	0	45,199	0	0
Other comprehensive income	12,667	0	0	0	0	12,667	0
Comprehensive income	57,866
Common stock issued:
Stock-based compensation plans, net	5,799	0	0	376	(19,173)	0	24,596
Purchase of treasury stock	(42,199)	0	0	0	0	0	(42,199)
Cash dividends:
Common stock ($0.09 per share in 2014 and $0.10 per share in 2015)	(14,639)	0	0		(14,639)	0	0
Preferred stock	(1,244)	0	0	0	(1,244)	0	0
Purchase of preferred stock	806	704	0	0	102	0	0
Stock-based compensation expense, net	4,412	0	0	4,412	0	0	0
Tax benefit of stock-based compensation	545	0	0	545	0	0	0
Ending balance at Mar. 31, 2014	2,901,024	61,158	1,750	1,623,323	1,402,549	(11,577)	(176,179)
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 common stock returned to authorized but unissued	(30,000)	0	(17)	(29,983)	0	0	0
Purchase of treasury stock	(4,105)	0	0	0	0	0	(4,105)
Cash dividends:
Common stock ($0.09 per share in 2014 and $0.10 per share in 2015)	(15,280)	0	0	0	(15,280)	0	0
Preferred stock	(1,228)	0	0	0	(1,228)	0	0
Stock-based compensation expense, net	5,774	0	0	5,774	0	0	0
Tax benefit of stock-based compensation	638	0	0	638	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)