Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss	Treasury Balance
Beginning Balance, shares at Dec. 31, 2014		344,174,653
Balance, beginning of period at Dec. 31, 2014	$ 2,909,286	$ 3,442	$ 1,265,257	$ 8,459,040	$ (514,943)	$ (6,303,510)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	752,207			752,207
Total other comprehensive income (loss), net of tax	(100,262)				(100,262)
Dividends	(249,262)			(249,262)
Repurchase of common stock	(1,537,020)					(1,537,020)
Share-based compensation and 401(k) match made with Treasury shares	40,851		39,457			1,394
Issuance of nonvested stock (in shares)		162,193
Issuance of nonvested stock	0	$ 2	(2)
Exercise of stock options (in shares)		518,858
Exercise of stock options	20,179	$ 5	20,174
Tax benefit of equity awards	3,675		3,675
Ending Balance, shares at Dec. 31, 2015		344,855,704
Balance, end of period at Dec. 31, 2015	1,839,654	$ 3,449	1,328,561	8,961,985	(615,205)	(7,839,136)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	692,164			692,164
Total other comprehensive income (loss), net of tax	49,824				49,824
Dividends	(252,321)			(252,321)
Repurchase of common stock	(465,341)					(465,341)
Share-based compensation and 401(k) match made with Treasury shares	39,826		36,956			2,870
Issuance of nonvested stock (in shares)		272,479
Issuance of nonvested stock	0	$ 2	(2)
Exercise of stock options (in shares)		466,871
Exercise of stock options	15,782	$ 5	15,777
Tax benefit of equity awards	$ 570		570
Retirement of treasury stock (in shares)	(165,000,000)	(165,000,000)
Retirement of treasury stock	$ 0	$ (1,650)		(8,064,155)		8,065,805
Ending Balance, shares at Dec. 31, 2016		180,595,054
Balance, end of period at Dec. 31, 2016	1,920,158	$ 1,806	1,381,862	1,337,673	(565,381)	(235,802)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	521,759			521,759
Total other comprehensive income (loss), net of tax	65,332				65,332
Dividends	(251,862)			(251,862)
Repurchase of common stock	(465,263)					(465,263)
Share-based compensation and 401(k) match made with Treasury shares	42,800		29,600			13,200
Issuance of nonvested stock (in shares)		408,950
Issuance of nonvested stock	$ 0	$ 4	(4)
Exercise of stock options (in shares)	282,000	282,543
Exercise of stock options	$ 11,353	$ 3	11,350
Ending Balance, shares at Dec. 31, 2017		181,286,547
Balance, end of period at Dec. 31, 2017	$ 1,844,277	$ 1,813	$ 1,422,808	$ 1,607,570	$ (500,049)	$ (687,865)