Financial instruments and derivatives - Additional Information (Details) BTU in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) BTU t	Dec. 31, 2018 USD ($)	Jan. 31, 2020 USD ($)
Derivative [Line Items]
Anticipated usage percentage of natural gas covered by a swap contract for the first succeeding fiscal year	61.00%
Anticipated usage percentage of aluminum covered by a swap contract for the first succeeding fiscal year	23.00%
Fair value of commodity cash flow hedges	$ (1,625,000)	$ (1,571,000)
Commodity gain (loss) expected to be reclassified to the income statement during the next 12 months	(1,578,000)
Fair value of foreign currency cash flow hedges	1,058,000	(1,712,000)
Foreign currency gain (loss) expected to be reclassified to the income statement during the next 12 months	1,057,000
Total fair value of other derivatives not designated as hedging instruments	$ 54,000	166,000
Natural gas swaps
Derivative [Line Items]
Approximate amount of commodity covered by swap contracts outstanding (btu) | BTU	4.4
Aluminum Swaps
Derivative [Line Items]
Approximate amount of commodity covered by swap contracts outstanding (tonne) | t	1,225
Foreign exchange forward contract
Derivative [Line Items]
Net position of derivative	$ 1,000	(305,000)
Cross currency swap | Subsequent event
Derivative [Line Items]
Notional amount			$ 250,000,000
Cross currency swap | U.S. Dollars | Subsequent event
Derivative [Line Items]
Fixed interest rate			5.75%
Cross currency swap | Euros | Subsequent event
Derivative [Line Items]
Fixed interest rate			3.856%
Accumulated Other Comprehensive Loss
Derivative [Line Items]
Amounts reclassified from accumulated other comprehensive loss	$ (107,000)	$ 88,000