Consolidated Statement of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Loss	Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle in period of adoption	$ 0		$ (254)	$ 254	$ 0
Balance, shares at Dec. 31, 2016		32,085				530
Balance, value at Dec. 31, 2016	460,880	$ 320	264,418	211,748	(7,721)	$ (7,885)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	3,996
Foreign currency translation adjustment	679				679
Adjustment to retirement benefit liability, net of taxes of $(2)	(3)				(3)
Adjustment to post employment health care benefit liability, net of taxes of $12	29				29
Stock compensation expense	1,635		1,635
Stock options exercised, shares		1
Stock options exercised	11	$ 0	11
Issuance of restricted stock	0
Net settlement of restricted stock units, shares		47				22
Net settlement of restricted stock units	(922)	$ 1	(1)			$ (922)
Balance, shares at Mar. 31, 2017		32,133				552
Balance, value at Mar. 31, 2017	466,305	$ 321	265,809	215,998	(7,016)	$ (8,807)
Balance, shares at Dec. 31, 2016		32,085				530
Balance, value at Dec. 31, 2016	460,880	$ 320	264,418	211,748	(7,721)	$ (7,885)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	37,384
Foreign currency translation adjustment	3,351
Adjustment to post employment health care benefit liability, net of taxes of $12	88
Balance, shares at Sep. 30, 2017		32,275				588
Balance, value at Sep. 30, 2017	505,431	$ 322	269,880	249,386	(4,290)	$ (9,867)
Balance, shares at Mar. 31, 2017		32,133				552
Balance, value at Mar. 31, 2017	466,305	$ 321	265,809	215,998	(7,016)	$ (8,807)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	12,769
Foreign currency translation adjustment	1,091				1,091
Adjustment to retirement benefit liability, net of taxes of $(2)	(3)				(3)
Adjustment to post employment health care benefit liability, net of taxes of $12	30				30
Stock compensation expense	1,556		1,556
Stock options exercised, shares		15
Stock options exercised	236	$ 0	236
Issuance of restricted stock (in shares)		2
Issuance of restricted stock	0
Net settlement of restricted stock units, shares		5				2
Net settlement of restricted stock units	(81)	$ 0	0			$ (81)
Balance, shares at Jun. 30, 2017		32,155				554
Balance, value at Jun. 30, 2017	481,903	$ 321	267,601	228,767	(5,898)	$ (8,888)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,619
Foreign currency translation adjustment	1,581				1,581
Adjustment to retirement benefit liability, net of taxes of $(2)	(2)				(2)
Adjustment to post employment health care benefit liability, net of taxes of $12	29				29
Stock compensation expense	1,878		1,878
Stock options exercised, shares		24
Stock options exercised	402	$ 0	402
Issuance of restricted stock	0
Net settlement of restricted stock units, shares		96				34
Net settlement of restricted stock units	(979)	$ 1	(1)			$ (979)
Balance, shares at Sep. 30, 2017		32,275				588
Balance, value at Sep. 30, 2017	505,431	$ 322	269,880	249,386	(4,290)	$ (9,867)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle in period of adoption	274		0	624	(350)
Balance, shares at Dec. 31, 2017		32,332				615
Balance, value at Dec. 31, 2017	531,719	$ 323	271,957	274,562	(4,366)	$ (10,757)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	8,352
Foreign currency translation adjustment	110				110
Adjustment to retirement benefit liability, net of taxes of $(2)	(5)				(5)
Adjustment to post employment health care benefit liability, net of taxes of $12	32				32
Stock compensation expense	2,097		2,097
Stock options exercised, shares		13
Stock options exercised	226	$ 0	226
Net settlement of restricted stock units, shares		53				24
Net settlement of restricted stock units	(850)	$ 1	(1)			$ (850)
Balance, shares at Mar. 31, 2018		32,398				639
Balance, value at Mar. 31, 2018	541,955	$ 324	274,279	283,538	(4,579)	$ (11,607)
Balance, shares at Dec. 31, 2017		32,332				615
Balance, value at Dec. 31, 2017	531,719	$ 323	271,957	274,562	(4,366)	$ (10,757)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	50,692
Foreign currency translation adjustment	(1,538)
Adjustment to post employment health care benefit liability, net of taxes of $12	95
Balance, shares at Sep. 30, 2018		32,842				778
Balance, value at Sep. 30, 2018	582,875	$ 328	280,149	325,878	(6,174)	$ (17,306)
Balance, shares at Mar. 31, 2018		32,398				639
Balance, value at Mar. 31, 2018	541,955	$ 324	274,279	283,538	(4,579)	$ (11,607)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,837
Foreign currency translation adjustment	(1,787)				(1,787)
Adjustment to retirement benefit liability, net of taxes of $(2)	(5)				(5)
Adjustment to post employment health care benefit liability, net of taxes of $12	31				31
Stock compensation expense	2,731		2,731
Stock options exercised, shares		21
Stock options exercised	300	$ 0	300
Issuance of restricted stock (in shares)		2
Issuance of restricted stock	0
Net settlement of restricted stock units, shares		334				128
Net settlement of restricted stock units	(5,166)	$ 3	(3)			$ (5,166)
Balance, shares at Jun. 30, 2018		32,755				767
Balance, value at Jun. 30, 2018	560,896	$ 327	277,307	306,375	(6,340)	$ (16,773)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	19,503
Foreign currency translation adjustment	139				139
Adjustment to retirement benefit liability, net of taxes of $(2)	(5)				(5)
Adjustment to post employment health care benefit liability, net of taxes of $12	32				32
Stock compensation expense	2,026		2,026
Stock options exercised, shares		50
Stock options exercised	817	$ 1	816
Net settlement of restricted stock units, shares		37				11
Net settlement of restricted stock units	(533)	$ 0	0			$ (533)
Balance, shares at Sep. 30, 2018		32,842				778
Balance, value at Sep. 30, 2018	$ 582,875	$ 328	$ 280,149	$ 325,878	$ (6,174)	$ (17,306)