Consolidated Statements of Changes in Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Capital In Excess of Par Value	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning balance, shares at Dec. 31, 2018		153,634
Beginning balance at Dec. 31, 2018	$ 1,593,147	$ 1,536	$ 1,106,641	$ 517,970	$ (33,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	372,689			372,689
Other comprehensive income (loss)	(2,780)				(2,780)
Purchase and retirement of common stock	(267,184)	$ (50)	(26,408)	(240,726)
Purchase and retirement of common stock, shares		(4,950)
Issuance of common stock under employee stock purchase plan	3,392	$ 1	3,391
Issuance of common stock under employee stock purchase plan, shares		77
Issuance of common stock upon exercise of stock options	38,472	$ 15	38,457
Issuance of common stock upon exercise of stock options, shares		1,461
Stock-based compensation	15,555		15,555
Dividends declared	(49,984)			(49,984)
Ending balance at Sep. 30, 2019	1,703,307	$ 1,502	1,137,636	599,949	(35,780)
Ending balance, shares at Sep. 30, 2019		150,222
Beginning balance, shares at Jun. 30, 2019		150,955
Beginning balance at Jun. 30, 2019	1,634,501	$ 1,509	1,122,068	541,664	(30,740)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	132,168			132,168
Other comprehensive income (loss)	(5,040)				(5,040)
Purchase and retirement of common stock	(81,367)	$ (15)	(7,469)	(73,883)
Purchase and retirement of common stock, shares		(1,400)
Issuance of common stock under employee stock purchase plan	999	$ 1	998
Issuance of common stock under employee stock purchase plan, shares		21
Issuance of common stock upon exercise of stock options	16,593	$ 7	16,586
Issuance of common stock upon exercise of stock options, shares		646
Stock-based compensation	5,453		5,453
Ending balance at Sep. 30, 2019	$ 1,703,307	$ 1,502	1,137,636	599,949	(35,780)
Ending balance, shares at Sep. 30, 2019		150,222
Beginning balance, shares at Dec. 31, 2019	149,745	149,745
Beginning balance at Dec. 31, 2019	$ 1,738,778	$ 1,497	1,158,900	601,885	(23,504)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	321,404			321,404
Other comprehensive income (loss)	(3,894)				(3,894)
Purchase and retirement of common stock	(325,644)	$ (62)	(34,999)	(290,583)
Purchase and retirement of common stock, shares		(6,185)
Issuance of common stock under employee stock purchase plan	3,401	$ 1	3,400
Issuance of common stock under employee stock purchase plan, shares		73
Issuance of common stock upon exercise of stock options	26,392	$ 9	26,383
Issuance of common stock upon exercise of stock options, shares		858
Stock-based compensation	20,458		20,458
Dividends declared	(51,462)			(51,462)
Ending balance at Sep. 30, 2020	$ 1,729,433	$ 1,445	1,174,142	581,244	(27,398)
Ending balance, shares at Sep. 30, 2020	144,491	144,491
Beginning balance, shares at Jun. 30, 2020		146,445
Beginning balance at Jun. 30, 2020	$ 1,708,625	$ 1,464	1,174,411	566,929	(34,179)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	111,096			111,096
Other comprehensive income (loss)	6,781				6,781
Purchase and retirement of common stock	(108,742)	$ (22)	(11,939)	(96,781)
Purchase and retirement of common stock, shares		(2,110)
Issuance of common stock under employee stock purchase plan	1,153	$ 1	1,152
Issuance of common stock under employee stock purchase plan, shares		26
Issuance of common stock upon exercise of stock options	4,053	$ 2	4,051
Issuance of common stock upon exercise of stock options, shares		130
Stock-based compensation	6,467		6,467
Ending balance at Sep. 30, 2020	$ 1,729,433	$ 1,445	$ 1,174,142	$ 581,244	$ (27,398)
Ending balance, shares at Sep. 30, 2020	144,491	144,491