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, 2019		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, shares		(6,185)
Purchase and retirement of common stock	(325,644)	$ (62)	(34,999)	(290,583)
Issuance of common stock under employee stock purchase plan, shares		73
Issuance of common stock under employee stock purchase plan	3,401	$ 1	3,400
Issuance of common stock upon exercise of stock options, shares		858
Issuance of common stock upon exercise of stock options	26,392	$ 9	26,383
Stock-based compensation	20,458		20,458
Dividends declared	(51,462)			(51,462)
Ending balance, shares at Sep. 30, 2020		144,491
Ending balance at Sep. 30, 2020	1,729,433	$ 1,445	1,174,142	581,244	(27,398)
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, shares		(2,110)
Purchase and retirement of common stock	(108,742)	$ (22)	(11,939)	(96,781)
Issuance of common stock under employee stock purchase plan, shares		26
Issuance of common stock under employee stock purchase plan	1,153	$ 1	1,152
Issuance of common stock upon exercise of stock options, shares		130
Issuance of common stock upon exercise of stock options	4,053	$ 2	4,051
Stock-based compensation	6,467		6,467
Ending balance, shares at Sep. 30, 2020		144,491
Ending balance at Sep. 30, 2020	$ 1,729,433	$ 1,445	1,174,142	581,244	(27,398)
Beginning balance, shares at Dec. 31, 2020	143,396	143,396
Beginning balance at Dec. 31, 2020	$ 1,739,907	$ 1,434	1,190,001	565,270	(16,798)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	401,293			401,293
Other comprehensive income (loss)	(3,745)				(3,745)
Purchase and retirement of common stock, shares		(5,218)
Purchase and retirement of common stock	(316,053)	$ (53)	(30,979)	(285,021)
Issuance of common stock under employee stock purchase plan, shares		64
Issuance of common stock under employee stock purchase plan	3,225	$ 1	3,224
Issuance of common stock upon exercise of stock options, shares		1,063
Issuance of common stock upon exercise of stock options	34,677	$ 11	34,666
Stock-based compensation	31,173		31,173
Dividends declared	$ (52,389)			(52,389)
Ending balance, shares at Sep. 30, 2021	139,305	139,305
Ending balance at Sep. 30, 2021	$ 1,838,088	$ 1,393	1,228,085	629,153	(20,543)
Beginning balance, shares at Jun. 30, 2021		141,027
Beginning balance at Jun. 30, 2021	1,805,606	$ 1,410	1,219,487	599,231	(14,522)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	138,045			138,045
Other comprehensive income (loss)	(6,021)				(6,021)
Purchase and retirement of common stock, shares		(1,980)
Purchase and retirement of common stock	(119,895)	$ (21)	(11,751)	(108,123)
Issuance of common stock under employee stock purchase plan, shares		22
Issuance of common stock under employee stock purchase plan	1,120	$ 1	1,119
Issuance of common stock upon exercise of stock options, shares		236
Issuance of common stock upon exercise of stock options	7,915	$ 3	7,912
Stock-based compensation	$ 11,318		11,318
Ending balance, shares at Sep. 30, 2021	139,305	139,305
Ending balance at Sep. 30, 2021	$ 1,838,088	$ 1,393	$ 1,228,085	$ 629,153	$ (20,543)