Components of accumulated other comprehensive loss - Reclassifications out of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Cost of goods sold	$ 348,935	$ 321,906
Selling, general and administrative expenses	10,702	9,625
Other operating expense, net	(973)	(881)
Income tax benefit (expense)	308	2,070
Net loss	(15,139)	(16,163)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Defined benefit plan and other postretirement liabilities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Cost of goods sold	1,307	777
Selling, general and administrative expenses	132	125
Other operating expense, net	394	350
Income tax benefit (expense)	(1,083)	(391)
Net loss	750	861
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized loss on financial instruments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Cost of goods sold	(46)	(46)
Income tax benefit (expense)	8	8
Net loss	$ (38)	$ (38)