Statements of Changes in Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Capital In Excess Of Par Value [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2017	$ 2,276,854	$ 12,356	$ 134,395	$ 2,130,103
Balance, in shares at Dec. 31, 2017		123,564
Net income	605,668			605,668
Share repurchases	(163,265)	$ (176)		(163,089)
Share repurchases, shares		(1,763)
Cash dividends declared	(42,594)			(42,594)
Share-based compensation and restricted share issuances, net of forfeitures	4,894	$ 6
Share-based compensation and restricted share issuances, net of forfeitures, shares		58
Share-based compensation and restricted share issuances, net of forfeitures			4,888
Taxes paid in exchange for shares withheld	(1,074)	$ (1)	(1,073)
Taxes paid in exchange for shares withheld, shares		(12)
Balance at Dec. 31, 2018	2,680,483	$ 12,185	138,210	2,530,088
Balance, in shares at Dec. 31, 2018		121,847
Net income	615,518			615,518
Share repurchases	(240,960)	$ (240)		(240,720)
Share repurchases, shares		(2,403)
Cash dividends declared	(54,584)			(54,584)
Reclassification of liability for modified equity awards	64,991		64,991
Share-based compensation and restricted share issuances, net of forfeitures	16,717	$ 10
Share-based compensation and restricted share issuances, net of forfeitures, shares		104
Share-based compensation and restricted share issuances, net of forfeitures			16,707
Taxes paid in exchange for shares withheld	(1,448)	$ (2)	(1,446)
Taxes paid in exchange for shares withheld, shares		(15)
Balance at Dec. 31, 2019	3,080,717	$ 11,953	218,462	2,850,302
Balance, in shares at Dec. 31, 2019		119,533
Net income	672,682			672,682
Share repurchases	(364,057)	$ (250)		(363,807)
Share repurchases, shares		(2,508)
Cash dividends declared	(71,046)			(71,046)
Cash paid for fractional shares	(612)	$ (1)	(611)
Cash paid for fractional shares, shares		(5)
Share-based compensation and restricted share issuances, net of forfeitures	11,337	$ 6
Share-based compensation and restricted share issuances, net of forfeitures, shares		57
Share-based compensation and restricted share issuances, net of forfeitures			11,331
Taxes paid in exchange for shares withheld	(2,733)	$ (2)	(2,731)
Taxes paid in exchange for shares withheld, shares		(19)
Balance at Dec. 31, 2020	$ 3,326,288	$ 11,706	$ 226,451	$ 3,088,131
Balance, in shares at Dec. 31, 2020		117,058