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			116,154			116,154
Other comprehensive income				3,907		3,907
Comprehensive income			116,154	3,907		120,061
Cash dividends declared ($0.72 per common share)			(57,881)			(57,881)
Common stock activity, long-term incentive plan	$ 747	21,612			(12,544)	9,815
Common stock activity, long-term incentive plan, Shares	224,399
Purchase of common stock under stock buyback program	$ (10,429)				(80,407)	(90,836)
Purchase of common stock under stock buyback program, Shares	(3,131,886)
Balance at Sep. 30, 2015	$ 258,138	1,710,903	781,769	(46,167)	(251,082)	2,453,561
Balance, Shares at Sep. 30, 2015	77,518,998
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			97,465			$ 97,465
Other comprehensive income				24,906		24,906
Comprehensive income			97,465	24,906		122,371
Cash dividends declared ($0.72 per common share)			(57,349)			(57,349)
Common stock activity, long-term incentive plan	$ 159	13,209			$ (3,227)	10,141
Common stock activity, long-term incentive plan, Shares	47,547
Purchase of common stock under stock buyback program, Shares					0
Issuance of stock from dividend reinvestment	$ 88	(163)			$ 896	821
Issuance of stock from dividend reinvestment, Shares	26,539
Balance at Sep. 30, 2016	$ 258,310	$ 1,697,147	$ 818,060	$ (55,689)	$ (228,701)	$ 2,489,127
Balance, Shares at Sep. 30, 2016	77,570,515					77,570,515