Derivative and hedging instruments (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Not Designated as Hedging Instrument [Member]	Dec. 31, 2012 Not Designated as Hedging Instrument [Member]	Sep. 30, 2013 Midwest Premium Contracts [Member] Not Designated as Hedging Instrument [Member] t	Dec. 31, 2012 Midwest Premium Contracts [Member] Not Designated as Hedging Instrument [Member] t	Sep. 30, 2013 Midwest Premium Contracts [Member] Prepaid and Other Current Assets [Member] Not Designated as Hedging Instrument [Member]		Dec. 31, 2012 Midwest Premium Contracts [Member] Prepaid and Other Current Assets [Member] Not Designated as Hedging Instrument [Member]		Sep. 30, 2013 Aluminum Sales Premium Contract Current [Member] Accrued and Other Current Liabilities [Member] Not Designated as Hedging Instrument [Member]	Dec. 31, 2012 Aluminum Sales Premium Contract Current [Member] Accrued and Other Current Liabilities [Member] Not Designated as Hedging Instrument [Member]	Sep. 30, 2013 E.ON Contingent Obligation [Member] Other Liabilities [Member] Not Designated as Hedging Instrument [Member]		Dec. 31, 2012 E.ON Contingent Obligation [Member] Other Liabilities [Member] Not Designated as Hedging Instrument [Member]		Sep. 30, 2013 Primary Aluminum Sales Contract Premium [Member] Not Designated as Hedging Instrument [Member] t		Dec. 31, 2012 Primary Aluminum Sales Contract Premium [Member] Not Designated as Hedging Instrument [Member] t
Fair value of derivative assets and liabilities by balance sheet location [Abstract]
DERIVATIVE ASSETS		$ 363	$ 0			$ 363	[1]	$ 0	[1]
DERIVATIVE LIABILITIES		771	16,539							771	1,170	0	[2]	15,369	[2]
Outstanding forward contracts not designated as hedging instruments [Abstract]
Aluminum forward contract outstanding (in metric tons)																5,993	[2]	20,400	[2]
Midwest premium contracts (in metric tons)				6,000	0
Accumulated other comprehensive loss to be reclassified to earnings over the next 12-month period	$ 153

[1] We entered into a fixed-price forward contract that settles monthly from January 2014 to March 2014 based on the Midwest Premium price published in the Platts Metals Week for the applicable period.

[2] Represents the remaining physical deliveries under the Glencore Metal Agreement.