Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ 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		47,437,000
Increase (Decrease) in Stockholders' Equity
Net income	51,335			51,335
Translation adjustment, net of tax	13,949				13,949
Options exercised	1,297	$ 0	1,297
Options exercised (in shares)		44,000
Stock-based compensation	10,337		10,337
Shares issued from release of Restricted Stock Units	(5,155)	$ 2	(5,157)
Shares issued from release of Restricted Stock Units (in shares)		208,000
Repurchase of common stock	(20,000)		(2,456)			$ (17,544)
Repurchase of common stock (in shares)		(425,000)
Cash dividends declared on common stock	(18,606)			(18,606)
Common stock issued	412		412
Common stock issued (in shares)		9,000
Balance at Jun. 30, 2017	899,411	$ 475	260,350	675,151	(19,021)	(17,544)
Balance (in shares) at Jun. 30, 2017		47,273,000
Balance at Dec. 31, 2016	865,842	$ 473	255,917	642,422	(32,970)
Balance (in shares) at Dec. 31, 2016		47,437,000
Increase (Decrease) in Stockholders' Equity
Net income	41,282			41,282
Translation adjustment, net of tax	7,469				7,469
Pension adjustment, net of tax	(944)				(944)
Options exercised	5,313	$ 3	5,310
Options exercised (in shares)		179,000
Stock-based compensation	2,228		2,228
Shares issued from release of Restricted Stock Units	(187)	$ 0	(187)
Shares issued from release of Restricted Stock Units (in shares)		6,000
Repurchase of common stock	(50,000)		(7,544)			(42,456)
Repurchase of common stock (in shares)		(713,000)
Retirement of common stock	0	$ (5)		(19,995)		20,000
Cash dividends declared on common stock	(19,794)			(19,794)
Balance at Dec. 31, 2017	884,778	$ 473	260,157	676,644	(12,496)	(40,000)
Balance (in shares) at Dec. 31, 2017		46,745,000
Increase (Decrease) in Stockholders' Equity
Net income	69,515			69,515
Translation adjustment, net of tax	(7,583)				(7,583)
Options exercised	695	$ 0	695
Options exercised (in shares)		23,000
Stock-based compensation	5,531		5,531
Tax benefit of options exercised	791		0	791
Shares issued from release of Restricted Stock Units	(5,111)	$ 2	(5,113)
Shares issued from release of Restricted Stock Units (in shares)		176,000
Repurchase of common stock	$ (25,439)		10,000			(35,439)
Repurchase of common stock (in shares)	(445,100)	(627,000)
Retirement of common stock	$ 0	$ (13)		(74,986)		74,999
Cash dividends declared on common stock	(19,840)			(19,840)
Common stock issued	465		465
Common stock issued (in shares)		8,000
Balance at Jun. 30, 2018	$ 903,802	$ 462	$ 271,735	$ 652,124	$ (20,079)	$ (440)
Balance (in shares) at Jun. 30, 2018		46,325,000