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	18,495			18,495
Other comprehensive income	6,325				6,325
Comprehensive income	24,820			18,495	6,325
Cash dividends declared ($0.24 per common share)	(20,586)			(20,586)
Common stock activity, long-term incentive plan, including excess income tax benefit	3,806	215	3,591
Common stock activity, long-term incentive plan, including excess income tax benefit, Shares		64,477
Balance at Mar. 31, 2012	2,375,203	282,284	1,638,225	474,879	(20,185)
Balance, shares at Mar. 31, 2012		84,769,973
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	48,576			48,576
Other comprehensive income	(8,553)				(8,553)
Comprehensive income	40,023			48,576	(8,553)
Cash dividends declared ($0.24 per common share)	(20,786)			(20,786)
Common stock activity, long-term incentive plan, including excess income tax benefit	4,585	114	4,471
Common stock activity, long-term incentive plan, including excess income tax benefit, Shares		34,174
Balance at Mar. 31, 2013	$ 2,477,100	$ 282,657	$ 1,652,109	$ 573,812	$ (31,478)
Balance, shares at Mar. 31, 2013		84,881,970