Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Total
Balances at Dec. 31, 2014	$ 1,671	$ 247,641	$ 2,555,972	$ (1,600,761)	$ 1,204,523
Comprehensive income:
Net earnings			427,235		427,235
Cash dividend declared and paid			(97,364)		(97,364)
Tax benefit of stock options exercised and restricted shares issued		12,877			12,877
Purchase of treasury shares				(262,275)	(262,275)
Share-based compensation		37,228			37,228
Stock option exercises and restricted share issuances, net of stock repurchased for payroll taxes		(29,018)		7,146	(21,872)
Balances at Dec. 31, 2015	1,671	268,728	2,885,843	(1,855,890)	1,300,352
Comprehensive income:
Net earnings			432,090		432,090
Cash dividend declared and paid			(98,990)		(98,990)
Tax benefit of stock options exercised and restricted shares issued		7,044			7,044
Purchase of treasury shares				(249,760)	(249,760)
Share-based compensation		40,625			40,625
Stock option exercises and restricted share issuances, net of stock repurchased for payroll taxes		(23,310)		6,010	(17,300)
Balances at Dec. 31, 2016	1,671	293,087	3,218,943	(2,099,640)	1,414,061
Comprehensive income:
Net earnings			686,263		686,263
Cash dividend declared and paid			(101,362)		(101,362)
Tax benefit of stock options exercised and restricted shares issued
Purchase of treasury shares				(179,813)	(179,813)
Share-based compensation		38,291			38,291
Restricted share issuances, net of stock repurchased for payroll taxes		(20,567)		2,452	(18,115)
Balances at Dec. 31, 2017	$ 1,671	$ 310,811	$ 3,803,844	$ (2,277,001)	$ 1,839,325