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	$ 274		$ 0	$ 624
Net income	31,189
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	31,189
Foreign currency translation adjustment	(1,677)				(1,677)
Adjustment to retirement benefit liability, net of taxes of $(4)	(10)				(10)
Adjustment to post employment health care benefit liability, net of taxes of $25	63				63
Stock compensation expense	4,828		4,828
Stock options exercised, shares		34
Stock options exercised	526	$ 0	526
Issuance of restricted stock (in shares)		2
Issuance of restricted stock	0	$ 0	0
Net settlement of restricted stock units, shares		387				152
Net settlement of restricted stock units	(6,016)	$ 4	(4)			$ (6,016)
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]
Cumulative effect of new accounting principle in period of adoption					$ (350)