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Derivative and hedging instruments (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2012
Mar. 31, 2012
Aluminum Sales Premium Contracts [Member]
T
Mar. 31, 2012
Designated as Hedging Instrument [Member]
Dec. 31, 2011
Designated as Hedging Instrument [Member]
Mar. 31, 2012
Primary Aluminum Put Option Contracts [Member]
Due From Affiliates [Member]
Designated as Hedging Instrument [Member]
Dec. 31, 2011
Primary Aluminum Put Option Contracts [Member]
Due From Affiliates [Member]
Designated as Hedging Instrument [Member]
Mar. 31, 2012
Primary Aluminum Put Option Contracts [Member]
Prepaid and Other Current Assets [Member]
Designated as Hedging Instrument [Member]
Dec. 31, 2011
Primary Aluminum Put Option Contracts [Member]
Prepaid and Other Current Assets [Member]
Designated as Hedging Instrument [Member]
Mar. 31, 2012
Power Contracts [Member]
Not Designated as Hedging Instrument [Member]
MWh
Dec. 31, 2011
Power Contracts [Member]
Not Designated as Hedging Instrument [Member]
MWh
Mar. 31, 2012
Power Contracts [Member]
Prepaid and Other Current Assets [Member]
Designated as Hedging Instrument [Member]
Dec. 31, 2011
Power Contracts [Member]
Prepaid and Other Current Assets [Member]
Designated as Hedging Instrument [Member]
Mar. 31, 2012
E.ON Contingent Obligation [Member]
Other Liabilities [Member]
Designated as Hedging Instrument [Member]
Dec. 31, 2011
E.ON Contingent Obligation [Member]
Other Liabilities [Member]
Designated as Hedging Instrument [Member]
Mar. 31, 2012
Aluminum Sales Premium Contract [Member]
Other Liabilities [Member]
Designated as Hedging Instrument [Member]
Dec. 31, 2011
Aluminum Sales Premium Contract [Member]
Other Liabilities [Member]
Designated as Hedging Instrument [Member]
Mar. 31, 2012
Aluminum Sales Premium Contract [Member]
Accrued and Other Current Liabilities [Member]
Designated as Hedging Instrument [Member]
Dec. 31, 2011
Aluminum Sales Premium Contract [Member]
Accrued and Other Current Liabilities [Member]
Designated as Hedging Instrument [Member]
Mar. 31, 2012
Natural Gas Forward Contracts [Member]
Accrued and Other Current Liabilities [Member]
Designated as Hedging Instrument [Member]
Dec. 31, 2011
Natural Gas Forward Contracts [Member]
Accrued and Other Current Liabilities [Member]
Designated as Hedging Instrument [Member]
Mar. 31, 2012
Primary Aluminum Sales Contract Premium [Member]
Not Designated as Hedging Instrument [Member]
t
Dec. 31, 2011
Primary Aluminum Sales Contract Premium [Member]
Not Designated as Hedging Instrument [Member]
t
Mar. 31, 2012
Primary Aluminum Put Option Contracts [Member]
Not Designated as Hedging Instrument [Member]
t
Dec. 31, 2011
Primary Aluminum Put Option Contracts [Member]
Not Designated as Hedging Instrument [Member]
t
Mar. 31, 2012
Primary Aluminum Put Option Contracts [Member]
Not Designated as Hedging Instrument [Member]
Related Party [Member]
t
Dec. 31, 2011
Primary Aluminum Put Option Contracts [Member]
Not Designated as Hedging Instrument [Member]
Related Party [Member]
t
Fair value of derivative assets and liabilities by balance sheet location [Abstract]                                                    
DERIVATIVE ASSETS     $ 3,034 $ 9,437 $ 1,801 $ 5,439 $ 1,180 $ 3,892     $ 53 $ 106                            
DERIVATIVE LIABILITIES     15,715 15,147                 14,310 13,958 350 301 819 607 236 281            
Outstanding forward contracts not designated as hedging instruments [Abstract]                                                    
Power contracts (in megawatt hours (MWH))                 1,886 [1] 3,772 [1]                                
Aluminum forward contract outstanding (in metric tons)                                         35,700 [2] 40,870 [2] 7,500 15,000 9,000 18,000
Primary aluminim physical delivery contract through Decmeber 31, 2013 (in metric tons per year)   20,400                                                
Accumulated other comprehensive loss to be reclassified to earnings over the next 12-month period $ 170                                                  
[1] Represents our expected usage during the remaining term of the Ravenswood power contract. In June 2011, the West Virginia PSC extended the term of this contract for an additional year.
[2] Represents the remaining physical deliveries under our 2013 Glencore Metal Agreement.