Consolidated Statements of Shareholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Adjustment to retirement benefit liability, taxes	$ 25	$ 9	$ 30
Adjustment to post-retirement healthcare benefit liability, taxes	99	830	$ 45
Unrealized loss on cash flow hedge	$ 82	$ 82