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
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle in period of adoption					$ (350)