Derivative Financial Instruments and Related Hedging Programs, Notional Quantity Table (Details)	Dec. 31, 2015 MMBTU mmlbs MWH
2016
Summary of material derivative positions
Percentage of natural gas purchases for which the Company's exposure to fluctuations in gas prices have been reduced	73.00%
2017
Summary of material derivative positions
Percentage of natural gas purchases for which the Company's exposure to fluctuations in gas prices have been reduced	72.00%
2018
Summary of material derivative positions
Percentage of natural gas purchases for which the Company's exposure to fluctuations in gas prices have been reduced	59.00%
Not Designated as Hedging Instrument | Aluminum | Option contracts | Purchase
Summary of material derivative positions
Derivative notional amount	4.7
Not Designated as Hedging Instrument | Aluminum | Swap contracts | Purchase
Summary of material derivative positions
Derivative notional amount	132.9
Not Designated as Hedging Instrument | Aluminum | Swap contracts | Sale
Summary of material derivative positions
Derivative notional amount	3.0
Not Designated as Hedging Instrument | Aluminum | Midwest premium swap contracts | Purchase
Summary of material derivative positions
Derivative notional amount	84.9
Not Designated as Hedging Instrument | Natural Gas | Swap contracts | Purchase
Summary of material derivative positions
Derivative notional amount | MMBTU	7,000,000
Not Designated as Hedging Instrument | Foreign Currency | Swap contracts | Purchase
Summary of material derivative positions
Derivative notional amount | MWH	4,699,750