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	210,308			210,308
Other comprehensive income (loss)	(10,675)				(10,675)
Purchase and retirement of common stock, shares		(4,075)
Purchase and retirement of common stock	(216,902)	$ (40)	(23,060)	(193,802)
Issuance of common stock under employee stock purchase plan, shares		47
Issuance of common stock under employee stock purchase plan	2,248	$ 0	2,248
Issuance of common stock upon exercise of stock options, shares		728
Issuance of common stock upon exercise of stock options	22,339	$ 7	22,332
Stock-based compensation	13,991		13,991
Dividends declared	(51,462)			(51,462)
Ending balance, shares at Jun. 30, 2020		146,445
Ending balance at Jun. 30, 2020	1,708,625	$ 1,464	1,174,411	566,929	(34,179)
Beginning balance, shares at Mar. 31, 2020		147,903
Beginning balance at Mar. 31, 2020	1,734,266	$ 1,479	1,170,649	597,486	(35,348)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	101,066			101,066
Other comprehensive income (loss)	1,169				1,169
Purchase and retirement of common stock, shares		(1,642)
Purchase and retirement of common stock	(89,470)	$ (16)	(9,293)	(80,161)
Issuance of common stock under employee stock purchase plan, shares		26
Issuance of common stock under employee stock purchase plan	1,142		1,142
Issuance of common stock upon exercise of stock options, shares		158
Issuance of common stock upon exercise of stock options	4,852	$ 1	4,851
Stock-based compensation	7,062		7,062
Dividends declared	(51,462)			(51,462)
Ending balance, shares at Jun. 30, 2020		146,445
Ending balance at Jun. 30, 2020	$ 1,708,625	$ 1,464	1,174,411	566,929	(34,179)
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	263,248			263,248
Other comprehensive income (loss)	2,276				2,276
Purchase and retirement of common stock, shares		(3,238)
Purchase and retirement of common stock	(196,158)	$ (32)	(19,228)	(176,898)
Issuance of common stock under employee stock purchase plan, shares		42
Issuance of common stock under employee stock purchase plan	2,105	$ 0	2,105
Issuance of common stock upon exercise of stock options, shares		827
Issuance of common stock upon exercise of stock options	26,762	$ 8	26,754
Stock-based compensation	19,855		19,855
Dividends declared	$ (52,389)			(52,389)
Ending balance, shares at Jun. 30, 2021	141,027	141,027
Ending balance at Jun. 30, 2021	$ 1,805,606	$ 1,410	1,219,487	599,231	(14,522)
Beginning balance, shares at Mar. 31, 2021		142,701
Beginning balance at Mar. 31, 2021	1,829,108	$ 1,427	1,208,433	634,651	(15,403)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	133,778			133,778
Other comprehensive income (loss)	881				881
Purchase and retirement of common stock, shares		(2,086)
Purchase and retirement of common stock	(129,219)	$ (21)	(12,389)	(116,809)
Issuance of common stock under employee stock purchase plan, shares		18
Issuance of common stock under employee stock purchase plan	966		966
Issuance of common stock upon exercise of stock options, shares		394
Issuance of common stock upon exercise of stock options	12,378	$ 4	12,374
Stock-based compensation	10,103		10,103
Dividends declared	$ (52,389)			(52,389)
Ending balance, shares at Jun. 30, 2021	141,027	141,027
Ending balance at Jun. 30, 2021	$ 1,805,606	$ 1,410	$ 1,219,487	$ 599,231	$ (14,522)