Consolidated Statements of Changes in Stockholders' Equity (USD $) In Thousands, except Share data	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), net [Member]
Balance at Dec. 31, 2011	$ 2,367,163	$ 282,069	$ 1,634,634	$ 476,970	$ (26,510)
Balance, Shares at Dec. 31, 2011		84,705,496
Net income	104,783			104,783
Other comprehensive income	14,723				14,723
Comprehensive income	119,506
Cash dividends declared	(61,915)			(61,915)
Common stock activity, long-term incentive plan	9,734	254	9,480
Common stock activity, long-term incentive plan, shares		76,398
Balance at Sep. 30, 2012	2,434,488	282,323	1,644,114	519,838	(11,787)
Balance, Shares at Sep. 30, 2012		84,781,894
Balance at Dec. 31, 2012	2,453,278	282,543	1,647,638	546,022	(22,925)
Balance, Shares at Dec. 31, 2012		84,847,796
Net income	128,640			128,640
Other comprehensive income	(61,846)				(61,846)
Comprehensive income	66,794			128,640	(61,846)
Cash dividends declared	(61,000)			(61,000)
Common stock activity, long-term incentive plan	12,370	256	12,114
Common stock activity, long-term incentive plan, shares		76,801
Purchase of common stock	(115,000)	(9,383)	(105,617)
Purchase of common stock, shares		(2,817,640)
Balance at Sep. 30, 2013	$ 2,356,442	$ 273,416	$ 1,554,135	$ 613,662	$ (84,771)
Balance, Shares at Sep. 30, 2013		82,106,957