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, 2017	$ 547,280	$ 448	$ 747,138	$ (3,368,182)	$ 3,433,282	$ (265,406)
Beginning balance, shares at Dec. 31, 2017		25,541,393
Stock Issued During Period, Value, Treasury Stock Reissued	(5,669)			(5,900)	(231)
Exercise of stock options and restricted stock units, shares		39,362
Treasury Stock, Value, Acquired, Cost Method	(118,750)			(118,750)
Repurchases of common stock, shares		(187,880)
Adjustment to Additional Paid in Capital, Share-Based Compensation	4,354		4,354
Net earnings	93,304				93,304
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	28,890					28,890
Ending balance at Mar. 31, 2018	560,747	$ 448	751,492	(3,481,032)	3,526,355	(236,516)
Ending balance, shares at Mar. 31, 2018		25,392,875
Beginning balance at Dec. 31, 2017	$ 547,280	$ 448	747,138	(3,368,182)	3,433,282	(265,406)
Beginning balance, shares at Dec. 31, 2017		25,541,393
Exercise of stock options and restricted stock units, shares	68,653
Treasury Stock, Value, Acquired, Cost Method	$ (237,500)
Repurchases of common stock, shares	(396,218)
Net earnings	$ 204,772
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(315)
Ending balance at Jun. 30, 2018	532,434	$ 448	755,374	(3,595,296)	3,637,629	(265,721)
Ending balance, shares at Jun. 30, 2018		25,213,828
Beginning balance at Mar. 31, 2018	560,747	$ 448	751,492	(3,481,032)	3,526,355	(236,516)
Beginning balance, shares at Mar. 31, 2018		25,392,875
Stock Issued During Period, Value, Treasury Stock Reissued	(4,291)			(4,485)	(194)
Exercise of stock options and restricted stock units, shares		29,291
Treasury Stock, Value, Acquired, Cost Method	(118,749)			(118,749)
Repurchases of common stock, shares		(208,338)
Adjustment to Additional Paid in Capital, Share-Based Compensation	3,882		3,882
Net earnings	111,468				111,468
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(29,205)					(29,205)
Ending balance at Jun. 30, 2018	532,434	$ 448	755,374	(3,595,296)	3,637,629	(265,721)
Ending balance, shares at Jun. 30, 2018		25,213,828
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	24,921,963
Stock Issued During Period, Value, Treasury Stock Reissued	$ (28,990)			(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	24,921,963
Exercise of stock options and restricted stock units, shares	226,595
Treasury Stock, Value, Acquired, Cost Method	$ (372,500)
Repurchases of common stock, shares	(539,326)
Net earnings	$ 238,965
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	981
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	24,609,232
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)			(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	24,609,232