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Derivatives, Hedging Programs and Other Financial Instruments, Realized and Unrealized Gains (Losses) Table (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
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Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Summary Of Realized And Unrealized Gains (Losses) |  |  |  |
Unrealized loss (gain) |  |  | $ (3.2) | $ (14.9)
Operational Risk Hedges | Not Designated as Hedging Instrument |  |  |  |
Summary Of Realized And Unrealized Gains (Losses) |  |  |  |
Unrealized loss (gain) | $ 11.9 | $ (10.9) | (3.2) | (14.9)
Cost of Sales [Member] | Alloy Metal Hedge [Member] | Not Designated as Hedging Instrument |  |  |  |
Summary Of Realized And Unrealized Gains (Losses) |  |  |  |
Realized (gain) loss | 0.2 | 0.0 | 0.1 | 0.0
Cost of Sales [Member] | Aluminum | Not Designated as Hedging Instrument |  |  |  |
Summary Of Realized And Unrealized Gains (Losses) |  |  |  |
Realized (gain) loss | (5.2) | 1.0 | (9.8) | 3.7
Cost of Sales [Member] | Operational Risk Hedges | Not Designated as Hedging Instrument |  |  |  |
Summary Of Realized And Unrealized Gains (Losses) |  |  |  |
Realized (gain) loss | (4.9) | 2.6 | (9.6) | 7.0
Cost of Sales [Member] | Natural Gas | Not Designated as Hedging Instrument |  |  |  |
Summary Of Realized And Unrealized Gains (Losses) |  |  |  |
Realized (gain) loss | 0.1 | 1.6 | 0.1 | 3.3
Gain (Loss) on Derivative Instruments | Aluminum | Not Designated as Hedging Instrument |  |  |  |
Summary Of Realized And Unrealized Gains (Losses) |  |  |  |
Unrealized loss (gain) | 11.6 | (6.9) | (4.7) | (10.0)
Gain (Loss) on Derivative Instruments | Natural Gas | Not Designated as Hedging Instrument |  |  |  |
Summary Of Realized And Unrealized Gains (Losses) |  |  |  |
Unrealized loss (gain) | $ 0.3 | $ (4.0) | $ 1.5 | $ (4.9)

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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959 + Details
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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 + Details
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