Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Capital In Excess of Par Value	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Stockholders' Equity	Noncontrolling Interest
Beginning balance (in shares) at Dec. 31, 2016		124,149,609
Beginning balance at Dec. 31, 2016	$ 150,410	$ 1,241	$ 22,018	$ 885,117	$ 3,811	$ (762,169)	$ 150,018	$ 392
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	55,786			55,719			55,719	67
Dividends declared	(9,058)			(9,058)			(9,058)
Currency translation adjustments	791				791		791
Derivative financial instruments	242				242		242
Defined benefit obligations	290				290		290
Available-for-sale investments	(108)				(108)		(108)
Exercise of stock options (in shares)		412,991
Exercise of stock options	9,793	$ 4	9,789				9,793
Shares placed in treasury	(5,193)		39,907			(45,100)	(5,193)
Stock-based compensation charges (in shares)		272,141
Stock-based compensation charges	3,412	$ 3	3,409				3,412
Distributions to noncontrolling interests	(146)							(146)
Ending balance (in shares) at Mar. 31, 2017		124,834,741
Ending balance at Mar. 31, 2017	$ 206,219	$ 1,248	75,123	931,778	5,026	(807,269)	205,906	313
Beginning balance (in shares) at Dec. 31, 2017	125,381,591	125,381,591
Beginning balance at Dec. 31, 2017	$ 285,757	$ 1,254	98,843	990,652	9,454	(814,809)	285,394	363
Recently adopted accounting standards at Dec. 31, 2017	9,926			12,171	(2,245)		9,926
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	66,413			66,441			66,441	(28)
Dividends declared	(16,174)			(16,174)			(16,174)
Currency translation adjustments	(3,124)				(3,124)		(3,124)
Derivative financial instruments	94				94		94
Defined benefit obligations	322				322		322
Available-for-sale investments	(66)				(66)		(66)
Exercise of stock options (in shares)		159,126
Exercise of stock options	3,807	$ 1	3,806				3,807
Shares placed in treasury	(5,940)					(5,940)	(5,940)
Stock-based compensation charges (in shares)		181,317
Stock-based compensation charges	4,461	$ 2	4,459				4,461
Distributions to noncontrolling interests	$ (226)							(226)
Ending balance (in shares) at Mar. 31, 2018	125,722,034	125,722,034
Ending balance at Mar. 31, 2018	$ 345,250	$ 1,257	$ 107,108	$ 1,053,090	$ 4,435	$ (820,749)	$ 345,141	$ 109