Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), Net [Member]	Treasury Stock [Member]	Total
Balance at Dec. 31, 2014	$ 267,820	$ 1,689,291	$ 723,496	$ (50,074)	$ (158,131)	$ 2,472,402
Balance, Shares at Dec. 31, 2014	80,426,485
Net income			74,988			74,988
Other comprehensive income				(10,010)		(10,010)
Comprehensive income			74,988	(10,010)		64,978
Cash dividends declared ($0.48 per common share)			(38,704)			(38,704)
Common stock activity, long-term incentive plan	$ 769	16,240			(10,375)	6,634
Common stock activity, long-term incentive plan, Shares	230,851
Purchase of common stock under stock buyback program	$ (8,537)				(66,733)	(75,270)
Purchase of common stock under stock buyback program, Shares	(2,563,607)
Balance at Jun. 30, 2015	$ 260,052	1,705,531	759,780	(60,084)	(235,239)	2,430,040
Balance, Shares at Jun. 30, 2015	78,093,729
Balance at Dec. 31, 2015	$ 258,063	1,684,101	777,944	(80,595)	(226,370)	$ 2,413,143
Balance, Shares at Dec. 31, 2015	77,496,429					77,496,429
Net income			50,746			$ 50,746
Other comprehensive income				31,054		31,054
Comprehensive income			50,746	31,054		81,800
Cash dividends declared ($0.48 per common share)			(38,238)			(38,238)
Common stock activity, long-term incentive plan	$ 134	9,584			(3,094)	6,624
Common stock activity, long-term incentive plan, Shares	40,392
Issuance of stock from dividend reinvestment	$ 5	(18)			49	36
Issuance of stock from dividend reinvestment, Shares	1,439
Balance at Jun. 30, 2016	$ 258,202	$ 1,693,667	$ 790,452	$ (49,541)	$ (229,415)	$ 2,463,365
Balance, Shares at Jun. 30, 2016	77,538,260					77,538,260