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Helguvik and Ravenswood Gains and (Losses) (Details) - USD ($)
$ in Millions

1 Months Ended

12 Months Ended

Jan. 31, 2017

Jun. 30, 2015

Dec. 31, 2018

Dec. 31, 2017

^[1],[2]

Dec. 31, 2016

Impaired Assets to be Disposed of by Method Other than Sale [Line Items]

Helguvik (gains) losses, impairment

$ 152.2

Helguvik (gains) losses

$ 4.5

$ 7.3

(152.2)

^[1]

Ravenswood Aluminum Smelter [Member]

Impaired Assets to be Disposed of by Method Other than Sale [Line Items]

Ravenswood charges

$ 30.9

$ 3.8

Proceeds from sale of Ravenswood facility and assets

$ 13.6

[1]	As adjusted due to the adoption of ASU 2016-18 "Statement of Cash Flows (Topic 230) Restricted Cash."
[2]	As adjusted due to the adoption of ASU 2017-07 Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost

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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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- Definition

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