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Derivatives, Hedging Programs and Other Financial Instruments, Material Derivative Positions (Details) mmlbs in Millions | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2017 EUR (€) MMBTU mmlbs
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Not Designated as Hedging Instrument | Option on Securities [Member] | Purchase |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount |  |  |  | 26.8
Not Designated as Hedging Instrument | Aluminum | Purchase |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount |  |  |  | 205.8
Not Designated as Hedging Instrument | Aluminum | Sales |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount |  |  |  | 1.4
Not Designated as Hedging Instrument | Natural Gas | Purchase |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount | MMBTU |  |  |  | 4,640,000.00
Not Designated as Hedging Instrument | Midwest premium swap contracts [Member] | Purchase |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount |  |  |  | 203.5
Designated as Hedging Instrument | Foreign Exchange Contract [Member] | Purchase |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Notional Amount | € |  |  |  | € 105,600
Designated as Hedging Instrument | Alloy Metal Hedge [Member] | Purchase |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount |  |  |  | 5.7
Scenario, Forecast [Member] | Natural Gas |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage | 61.00% | 72.00% | 74.00% |
Scenario, Forecast [Member] | Electricity Member |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage |  |  | 54.00% |

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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition The percentage of expected requirements covered by the aggregate notional amount of derivative contracts expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract as a percentage of expected need. + References No definition available. + Details
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