Interim Consolidated Statements of Shareholders' Equity and Comprehensive Income (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2018	$ 590,063	$ 448	$ 764,717	$ (3,814,604)	$ 3,941,916	$ (302,414)
Beginning balance, shares at Dec. 31, 2018		24,921,963
Stock Issued During Period, Value, Treasury Stock Reissued	(28,990)		(751)	(28,257)	(18)
Exercise of stock options and restricted stock units, shares		171,752
Treasury Stock, Value, Acquired, Cost Method	(186,250)			(186,250)
Repurchases of common stock, shares		(290,429)
Adjustment to Additional Paid in Capital, Share-Based Compensation	4,482		4,482
Net earnings	111,805				111,805
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	12,660					12,660
Ending balance at Mar. 31, 2019	561,750	$ 448	769,950	(3,972,597)	4,053,703	(289,754)
Ending balance, shares at Mar. 31, 2019		24,803,286
Beginning balance at Dec. 31, 2018	$ 590,063	$ 448	764,717	(3,814,604)	3,941,916	(302,414)
Beginning balance, shares at Dec. 31, 2018		24,921,963
Exercise of stock options and restricted stock units, shares	226,841
Treasury Stock, Value, Acquired, Cost Method	$ (558,700)
Repurchases of common stock, shares	(794,603)
Net earnings	$ 368,360
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(12,572)
Ending balance at Sep. 30, 2019	438,301	$ 448	778,868	(4,335,747)	4,309,718	(314,986)
Ending balance, shares at Sep. 30, 2019		24,354,201
Beginning balance at Mar. 31, 2019	561,750	$ 448	769,950	(3,972,597)	4,053,703	(289,754)
Beginning balance, shares at Mar. 31, 2019		24,803,286
Stock Issued During Period, Value, Treasury Stock Reissued	(8,767)		0	(9,307)	(540)
Exercise of stock options and restricted stock units, shares		54,843
Treasury Stock, Value, Acquired, Cost Method	(186,250)			(186,250)
Repurchases of common stock, shares		(248,897)
Adjustment to Additional Paid in Capital, Share-Based Compensation	4,338		4,338
Net earnings	127,160				127,160
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(11,679)					(11,679)
Ending balance at Jun. 30, 2019	504,086	$ 448	774,288	(4,149,540)	4,180,323	(301,433)
Ending balance, shares at Jun. 30, 2019		24,609,232
Stock Issued During Period, Value, Treasury Stock Reissued	(160)		(117)	(43)	0
Exercise of stock options and restricted stock units, shares		246
Treasury Stock, Value, Acquired, Cost Method	(186,250)			(186,250)
Repurchases of common stock, shares		(255,277)
Adjustment to Additional Paid in Capital, Share-Based Compensation	4,463		4,463
Net earnings	129,395				129,395
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(13,553)					(13,553)
Ending balance at Sep. 30, 2019	438,301	$ 448	778,868	(4,335,747)	4,309,718	(314,986)
Ending balance, shares at Sep. 30, 2019		24,354,201
Beginning balance at Dec. 31, 2019	$ 420,780	$ 448	783,871	(4,539,154)	4,499,288	(323,673)
Beginning balance, shares at Dec. 31, 2019	24,125,317	24,125,317
Stock Issued During Period, Value, Treasury Stock Reissued	$ (7,135)		0	(9,355)	(2,220)
Exercise of stock options and restricted stock units, shares		50,372
Treasury Stock, Value, Acquired, Cost Method	(200,000)			(200,000)
Repurchases of common stock, shares		(268,161)
Adjustment to Additional Paid in Capital, Share-Based Compensation	4,395		4,395
Net earnings	98,115				98,115
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(24,028)					(24,028)
Ending balance at Mar. 31, 2020	306,397	$ 448	788,266	(4,729,799)	4,595,183	(347,701)
Ending balance, shares at Mar. 31, 2020		23,907,528
Beginning balance at Dec. 31, 2019	$ 420,780	$ 448	783,871	(4,539,154)	4,499,288	(323,673)
Beginning balance, shares at Dec. 31, 2019	24,125,317	24,125,317
Exercise of stock options and restricted stock units, shares	144,776
Treasury Stock, Value, Acquired, Cost Method	$ (400,000)
Repurchases of common stock, shares	(475,530)
Net earnings	$ 386,444
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(3,600)
Ending balance at Sep. 30, 2020	$ 443,067	$ 448	799,701	(4,912,096)	4,882,287	(327,273)
Ending balance, shares at Sep. 30, 2020	23,794,563	23,794,563
Beginning balance at Mar. 31, 2020	$ 306,397	$ 448	788,266	(4,729,799)	4,595,183	(347,701)
Beginning balance, shares at Mar. 31, 2020		23,907,528
Stock Issued During Period, Value, Treasury Stock Reissued	(10,615)		0	(11,837)	(1,222)
Exercise of stock options and restricted stock units, shares		63,737
Adjustment to Additional Paid in Capital, Share-Based Compensation	4,423		4,423
Net earnings	126,562				126,562
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	2,096					2,096
Ending balance at Jun. 30, 2020	450,093	$ 448	792,689	(4,717,962)	4,720,523	(345,605)
Ending balance, shares at Jun. 30, 2020		23,971,265
Stock Issued During Period, Value, Treasury Stock Reissued	(8,444)		(2,582)	(5,865)	(3)
Exercise of stock options and restricted stock units, shares		30,667
Treasury Stock, Value, Acquired, Cost Method	(199,999)			(199,999)
Repurchases of common stock, shares		(207,369)
Adjustment to Additional Paid in Capital, Share-Based Compensation	4,430		4,430
Net earnings	161,767				161,767
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	18,332					18,332
Ending balance at Sep. 30, 2020	$ 443,067	$ 448	$ 799,701	$ (4,912,096)	$ 4,882,287	$ (327,273)
Ending balance, shares at Sep. 30, 2020	23,794,563	23,794,563