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, 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		$ 124	$ 1,064,601	$ 3,507	$ 37,033
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock		1	(1)
Repurchases of common stock		(4)			(223,440)
Stock-based compensation expense			33,000
Foreign currency translation adjustments, net of tax	$ (13,299)			(13,299)
Net income	320,724				320,724
Cash dividends ($.31 per share, $.28 per share, $.93 per share, and $.84 per share)					(103,004)
Balance at end of period at Sep. 30, 2018		$ 121	1,097,600	(9,792)	31,313
Balance at beginning of period (in shares) at Jun. 30, 2018		122,445
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock (in shares)		(2)
Repurchases of common stock (in shares)		(1,068)
Balance at end of period (in shares) at Sep. 30, 2018		121,375
Balance at beginning of period at Jun. 30, 2018		$ 122	1,086,545	(6,409)	29,240
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock		0	0
Repurchases of common stock		(1)			(78,958)
Stock-based compensation expense			11,055
Foreign currency translation adjustments, net of tax	(3,383)			(3,383)
Net income	115,242				115,242
Cash dividends ($.31 per share, $.28 per share, $.93 per share, and $.84 per share)					(34,211)
Balance at end of period at Sep. 30, 2018		$ 121	1,097,600	(9,792)	31,313
Balance at beginning of period (in shares) at Dec. 31, 2018		119,078
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock (in shares)		549
Repurchases of common stock (in shares)		(3,530)
Balance at end of period (in shares) at Sep. 30, 2019		116,097
Balance at beginning of period at Dec. 31, 2018	1,063,198	$ 119	1,079,188	(16,109)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock		1	(1)
Repurchases of common stock		(4)			(208,166)
Stock-based compensation expense			34,803
Foreign currency translation adjustments, net of tax	(9,762)			(9,762)
Net income	341,591				341,591
Cash dividends ($.31 per share, $.28 per share, $.93 per share, and $.84 per share)					(109,830)
Balance at end of period at Sep. 30, 2019	1,111,830	$ 116	1,113,990	(25,871)	23,595
Balance at beginning of period (in shares) at Jun. 30, 2019		117,561
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock (in shares)		(3)
Repurchases of common stock (in shares)		(1,461)
Balance at end of period (in shares) at Sep. 30, 2019		116,097
Balance at beginning of period at Jun. 30, 2019		$ 118	1,102,101	(15,860)	22,868
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock		0	0
Repurchases of common stock		(2)			(80,219)
Stock-based compensation expense			11,889
Foreign currency translation adjustments, net of tax	(10,011)			(10,011)
Net income	117,181				117,181
Cash dividends ($.31 per share, $.28 per share, $.93 per share, and $.84 per share)					(36,235)
Balance at end of period at Sep. 30, 2019	$ 1,111,830	$ 116	$ 1,113,990	$ (25,871)	$ 23,595