Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Balance at Dec. 31, 2016	$ 865,842	$ 473	$ 255,917	$ 642,422	$ (32,970)
Balance (in shares) at Dec. 31, 2016		47,437
Increase (Decrease) in Stockholders' Equity
Net income	79,532			79,532
Translation adjustment, net of tax	19,492				19,492
Options exercised	3,566	$ 1	3,565
Options exercised (in shares)		120
Stock-based compensation	10,764		10,764
Shares issued from release of Restricted Stock Units	(5,166)	$ 2	(5,168)
Shares issued from release of Restricted Stock Units (in shares)		210
Repurchase of common stock	(20,000)					$ (20,000)
Repurchase of common stock (in shares)		(461)
Cash dividends declared on common stock	(38,400)			(38,400)
Common stock issued	412		412
Common stock issued (in shares)		9
Balance at Sep. 30, 2017	916,042	$ 476	265,490	683,554	(13,478)	(20,000)
Balance (in shares) at Sep. 30, 2017		47,315
Balance at Dec. 31, 2016	865,842	$ 473	255,917	642,422	(32,970)
Balance (in shares) at Dec. 31, 2016		47,437
Increase (Decrease) in Stockholders' Equity
Net income	13,085			13,085
Translation adjustment, net of tax	1,926				1,926
Pension adjustment, net of tax	(944)				(944)
Options exercised	3,044	$ 2	3,042
Options exercised (in shares)		103
Stock-based compensation	1,801		1,801
Shares issued from release of Restricted Stock Units	(176)	$ 0	(176)
Shares issued from release of Restricted Stock Units (in shares)		4
Repurchase of common stock	(50,000)		(10,000)			(40,000)
Repurchase of common stock (in shares)		(677)
Retirement of common stock	0	$ (5)		(19,995)		20,000
Balance at Dec. 31, 2017	884,778	$ 473	260,157	676,644	(12,496)	(40,000)
Balance (in shares) at Dec. 31, 2017		46,745
Increase (Decrease) in Stockholders' Equity
Net income	113,876			113,876
Translation adjustment, net of tax	(10,533)				(10,533)
Options exercised	695	$ 0	695
Options exercised (in shares)		23
Stock-based compensation	7,939		7,939
Tax benefit of options exercised	791		0	791
Shares issued from release of Restricted Stock Units	(5,128)	$ 2	(5,130)
Shares issued from release of Restricted Stock Units (in shares)		177
Repurchase of common stock	(49,490)		10,000			(59,490)
Repurchase of common stock (in shares)		(985)
Retirement of common stock	0	$ (13)		(74,986)		74,999
Cash dividends declared on common stock	(29,974)			(29,974)
Common stock issued	465		465
Common stock issued (in shares)		8
Balance at Sep. 30, 2018	$ 913,419	$ 462	$ 274,126	$ 686,351	$ (23,029)	$ (24,491)
Balance (in shares) at Sep. 30, 2018		45,968