Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss, net [Member]	Treasury Stock [Member]
Balance at Dec. 31, 2012	$ 2,453,278	$ 282,543	$ 1,668,517	$ 546,022	$ (22,925)	$ (20,879)
Balance, Shares at Dec. 31, 2012		84,847,796
Net income	163,356			163,356
Other comprehensive income	(12,454)				(12,454)
Cash dividends declared ($0.96 per common share)	(81,212)			(81,212)
Common stock activity, long-term incentive plan	17,101	$ 690	(21,050)			37,461
Common stock activity, long-term incentive plan, shares		207,006
Purchase of common stock under stock buyback program	(115,000)	$ (9,383)				(105,617)
Purchase of common stock under stock buyback program, shares		(2,817,640)
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	175,722			175,722
Other comprehensive income	(14,695)				(14,695)
Cash dividends declared ($0.96 per common share)	(80,392)			(80,392)
Common stock activity, long-term incentive plan	14,316	$ 1,029	41,824			(28,537)
Common stock activity, long-term incentive plan, shares		309,087
Purchase of common stock under stock buyback program	(47,618)	$ (7,059)				(40,559)
Purchase of common stock under stock buyback program, shares		(2,119,764)
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	131,461			131,461
Other comprehensive income	(30,521)				(30,521)
Cash dividends declared ($0.96 per common share)	(77,013)			(77,013)
Common stock activity, long-term incentive plan	12,427	$ 1,248	(5,190)			16,369
Common stock activity, long-term incentive plan, shares		374,944
Purchase of common stock under stock buyback program	(95,613)	$ (11,005)				(84,608)
Purchase of common stock under stock buyback program, shares		(3,305,000)
Balance at Dec. 31, 2015	$ 2,413,143	$ 258,063	$ 1,684,101	$ 777,944	$ (80,595)	$ (226,370)
Balance, Shares at Dec. 31, 2015		77,496,429