Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), net [Member]	Treasury Stock [Member]
Balance at Dec. 31, 2013	$ 2,425,069	$ 273,850	$ 1,647,467	$ 628,166	$ (35,379)	$ (89,035)
Balance, Shares at Dec. 31, 2013		82,237,162
Net income	89,077			89,077
Other comprehensive income	12,088				12,088
Comprehensive income	101,165			89,077	12,088
Cash dividends declared ($0.48 per common share)	(40,301)			(40,301)
Common stock activity, long-term incentive plan	6,649	$ 710	44,582			(38,643)
Common stock activity, long-term incentive plan, shares		213,359
Purchase of common stock under stock buyback program		$ (1,965)				1,965
Purchase of common stock under stock buyback program, shares		(590,222)
Balance at Jun. 30, 2014	2,492,582	$ 272,595	1,692,049	676,942	(23,291)	(125,713)
Balance, Shares at Jun. 30, 2014		81,860,299
Balance at Dec. 31, 2014	2,472,402	$ 267,820	1,689,291	723,496	(50,074)	(158,131)
Balance, Shares at Dec. 31, 2014		80,426,485
Net income	74,988			74,988
Other comprehensive income	(10,010)				(10,010)
Comprehensive income	64,978			74,988	(10,010)
Cash dividends declared ($0.48 per common share)	(38,704)			(38,704)
Common stock activity, long-term incentive plan	6,634	$ 769	16,240			(10,375)
Common stock activity, long-term incentive plan, shares		230,851
Purchase of common stock under stock buyback program	(75,270)	$ (8,537)				(66,733)
Purchase of common stock under stock buyback program, shares		(2,563,607)
Balance at Jun. 30, 2015	$ 2,430,040	$ 260,052	$ 1,705,531	$ 759,780	$ (60,084)	$ (235,239)
Balance, Shares at Jun. 30, 2015		78,093,729