Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	COMMON STOCK	CAPITAL SURPLUS	ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS)	RETAINED EARNINGS
Balance at beginning of period (in shares) at Dec. 31, 2016		127,797
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock (in shares)		818
Repurchases of common stock (in shares)		(3,268)
Balance at end of period (in shares) at Sep. 30, 2017		125,347
Balance at beginning of period at Dec. 31, 2016		$ 128	$ 1,022,411	$ (20,502)	$ 84,562
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock		1	(1)
Net income	$ 243,537				243,537
Repurchases of common stock		(4)			(152,650)
Stock-based compensation expense			31,163
Foreign currency translation adjustments, net of tax	22,206			22,206
Cash dividends ($.84 per share and $.72 per share)					(91,033)
Balance at end of period at Sep. 30, 2017		$ 125	1,053,573	1,704	84,416
Balance at beginning of period (in shares) at Dec. 31, 2017		124,261
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock (in shares)		523
Repurchases of common stock (in shares)		(3,409)
Balance at end of period (in shares) at Sep. 30, 2018		121,375
Balance at beginning of period at Dec. 31, 2017	1,105,265	$ 124	1,064,601	3,507	37,033
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock		1	(1)
Net income	320,724				320,724
Repurchases of common stock		(4)			(223,440)
Stock-based compensation expense			33,000
Foreign currency translation adjustments, net of tax	(13,299)			(13,299)
Cash dividends ($.84 per share and $.72 per share)					(103,004)
Balance at end of period at Sep. 30, 2018	$ 1,119,242	$ 121	$ 1,097,600	$ (9,792)	$ 31,313