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	57,799			57,799
Other comprehensive income	9,048				9,048
Comprehensive income	66,847			57,799	9,048
Cash dividends declared ($0.48 per common share)	(41,252)			(41,252)
Common stock activity, long-term incentive plan	6,604	228	6,376
Common stock activity, long-term incentive plan,shares		68,485
Balance at Jun. 30, 2012	2,399,362	282,297	1,641,010	493,517	(17,462)
Balance, Shares at Jun. 30, 2012		84,773,981
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	95,438			95,438
Other comprehensive income	(55,770)				(55,770)
Comprehensive income	39,668			95,438	(55,770)
Cash dividends declared ($0.48 per common share)	(40,894)			(40,894)
Common stock activity, long-term incentive plan	8,288	159	8,129
Common stock activity, long-term incentive plan,shares		47,621
Purchase of common stock	(115,000)	(9,383)	(105,617)
Purchase of common stock, shares		(2,817,640)
Balance at Jun. 30, 2013	$ 2,345,340	$ 273,319	$ 1,550,150	$ 600,566	$ (78,695)
Balance, Shares at Jun. 30, 2013		82,077,777