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	240,521			240,521
Other comprehensive income (loss)	2,260				2,260
Purchase and retirement of common stock	(185,817)	$ (35)	(18,939)	(166,843)
Purchase and retirement of common stock, shares		(3,550)
Issuance of common stock under employee stock purchase plan	2,393	$ 0	2,393
Issuance of common stock under employee stock purchase plan, shares		56
Issuance of common stock upon exercise of stock options	21,879	$ 8	21,871
Issuance of common stock upon exercise of stock options, shares		815
Stock-based compensation	10,102		10,102
Dividends declared	(49,984)			(49,984)
Ending balance at Jun. 30, 2019	1,634,501	$ 1,509	1,122,068	541,664	(30,740)
Ending balance, shares at Jun. 30, 2019		150,955
Beginning balance, shares at Mar. 31, 2019		152,276
Beginning balance at Mar. 31, 2019	1,636,550	$ 1,523	1,111,366	552,381	(28,720)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	126,540			126,540
Other comprehensive income (loss)	(2,020)				(2,020)
Purchase and retirement of common stock	(97,028)	$ (18)	(9,737)	(87,273)
Purchase and retirement of common stock, shares		(1,825)
Issuance of common stock under employee stock purchase plan	1,071	$ 0	1,071
Issuance of common stock under employee stock purchase plan, shares		24
Issuance of common stock upon exercise of stock options	14,308	$ 4	14,304
Issuance of common stock upon exercise of stock options, shares		480
Stock-based compensation	5,064		5,064
Dividends declared	(49,984)			(49,984)
Ending balance at Jun. 30, 2019	$ 1,634,501	$ 1,509	1,122,068	541,664	(30,740)
Ending balance, shares at Jun. 30, 2019		150,955
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	210,308			210,308
Other comprehensive income (loss)	(10,675)				(10,675)
Purchase and retirement of common stock	(216,902)	$ (40)	(23,060)	(193,802)
Purchase and retirement of common stock, shares		(4,075)
Issuance of common stock under employee stock purchase plan	2,248	$ 0	2,248
Issuance of common stock under employee stock purchase plan, shares		47
Issuance of common stock upon exercise of stock options	22,339	$ 7	22,332
Issuance of common stock upon exercise of stock options, shares		728
Stock-based compensation	13,991		13,991
Dividends declared	(51,462)			(51,462)
Ending balance at Jun. 30, 2020	$ 1,708,625	$ 1,464	1,174,411	566,929	(34,179)
Ending balance, shares at Jun. 30, 2020	146,445	146,445
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	(89,470)	$ (16)	(9,293)	(80,161)
Purchase and retirement of common stock, shares		(1,642)
Issuance of common stock under employee stock purchase plan	1,142	$ 0	1,142
Issuance of common stock under employee stock purchase plan, shares		26
Issuance of common stock upon exercise of stock options	4,852	$ 1	4,851
Issuance of common stock upon exercise of stock options, shares		158
Stock-based compensation	7,062		7,062
Dividends declared	(51,462)			(51,462)
Ending balance at Jun. 30, 2020	$ 1,708,625	$ 1,464	$ 1,174,411	$ 566,929	$ (34,179)
Ending balance, shares at Jun. 30, 2020	146,445	146,445