Consolidated Statements of Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
tax provision (benefit) for actuarial (gain) loss	$ 401	$ 603	$ (531)
tax provision for amortization of deferred loss	0	48	10
tax provision for amortization of prior service cost	30	30	30
tax provision for settlements and plan changes	$ 0	$ 0	$ 91