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)		829
Repurchases of common stock, (in shares)		(2,442)
Balance at end of period, (in shares) at Jun. 30, 2017		126,184
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	$ 158,837				158,837
Repurchases of common stock		(3)			(115,807)
Stock-based compensation expense			20,727
Foreign currency translation adjustments, net of tax	14,996			14,996
Cash dividends ($.56 per share and $.48 per share)					(60,965)
Balance at end of period at Jun. 30, 2017		$ 126	1,043,137	(5,506)	66,627
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)		525
Repurchases of common stock, (in shares)		(2,341)
Balance at end of period, (in shares) at Jun. 30, 2018		122,445
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	205,482				205,482
Repurchases of common stock		(3)			(144,482)
Stock-based compensation expense			21,945
Foreign currency translation adjustments, net of tax	(9,916)			(9,916)
Cash dividends ($.56 per share and $.48 per share)					(68,793)
Balance at end of period at Jun. 30, 2018	$ 1,109,498	$ 122	$ 1,086,545	$ (6,409)	$ 29,240