Segment and Geographical Area Information, Textuals (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information
Non-cash LIFO inventory benefits	$ 24.1	$ 4.9	$ 7.1
Environmental expense	4.5	1.2	3.9
Asset impairment charge	0	4.4	0
Unrealized gains (losses) on derivatives	3.9	16.0	(25.9)
Non-cash defined benefit net periodic benefit income	(22.0)	(11.5)	(5.7)
Fabricated Products
Segment Reporting Information
Non-cash LIFO inventory benefits	24.1	4.9	7.1
Environmental expense	4.0	1.1	1.7
Asset impairment charge	0		0
Unrealized gains (losses) on derivatives	0.7	15.2	(29.9)
United States
Segment Reporting Information
Number of production facilities	11
Canada
Segment Reporting Information
Number of production facilities	1
Anglesey Aluminium Limited
Segment Reporting Information
Ownership percentage of Anglesey	49.00%
VEBAs
Segment Reporting Information
Non-cash defined benefit net periodic benefit income	(22.5)	(11.9)	(6.0)
Idled equipment \| Fabricated Products
Segment Reporting Information
Asset impairment charge		$ 4.4

Number of Production Facilities.

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- Definition

Increase (decrease) in the accrual for environmental loss contingencies during the reporting period.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 5.Y.Q2)

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- Definition

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.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

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The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (h)

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

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- Definition

The change in the inventory reserve representing the cumulative difference in cost between the first in, first out and the last in, first out inventory valuation methods, which change has been reflected in the statement of income during the period.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.6b)

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-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 5

-Section L

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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.

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