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	23,121			23,121
Translation adjustment, net of tax	3,322				3,322
Options exercised	314	$ 0	314
Options exercised (in shares)		11,000
Stock-based compensation	7,650		7,650
Shares issued from release of Restricted Stock Units	(5,124)	$ 2	(5,126)
Shares issued from release of Restricted Stock Units (in shares)		197,000
Cash dividends declared on common stock	(8,584)			(8,584)
Common stock issued	412		412
Common stock issued (in shares)		9,000
Balance at Mar. 31, 2017	886,953	$ 475	259,167	656,959	(29,648)	$ 0
Balance (in shares) at Mar. 31, 2017		47,654,000
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	69,496			69,496
Translation adjustment, net of tax	18,096				18,096
Pension adjustment, net of tax	(944)				(944)
Options exercised	6,296	$ 3	6,293
Options exercised (in shares)		212,000
Stock-based compensation	4,915		4,915
Shares issued from release of Restricted Stock Units	(218)	$ 0	(218)
Shares issued from release of Restricted Stock Units (in shares)		17,000
Repurchase of common stock	(70,000)		(10,000)			(60,000)
Repurchase of common stock (in shares)		(1,138,000)
Retirement of common stock	0	$ (5)		(19,995)		20,000
Cash dividends declared on common stock	(29,816)			(29,816)
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	25,429			25,429
Translation adjustment, net of tax	3,860				3,860
Options exercised	695	$ 0	695
Options exercised (in shares)		23,000
Stock-based compensation	2,716		2,716
Tax benefit of options exercised	792		0	792
Shares issued from release of Restricted Stock Units	(5,027)	$ 2	(5,029)
Shares issued from release of Restricted Stock Units (in shares)		163,000
Repurchase of common stock	$ (24,999)		10,000			(34,999)
Repurchase of common stock (in shares)	(437,500)	(620,000)
Cash dividends declared on common stock	$ (9,647)			(9,647)
Common stock issued	465		465
Common stock issued (in shares)		8,000
Balance at Mar. 31, 2018	$ 879,062	$ 475	$ 269,004	$ 693,218	$ (8,636)	$ (74,999)
Balance (in shares) at Mar. 31, 2018		46,319,000