Segment and Geographical Area Information, Textuals (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
(Decrease) increase in Fabricated Products segment operating income relating to primary aluminum hedging activities		$ (25.0)	$ 3.1
Increase in Fabricated Products segment asset related to primary aluminum hedging activities		0.6
Net non-cash LIFO (benefits) charges	(4.9)	(7.1)	16.5
Environmental expense	1.2	3.9	13.9
Impairment charge	4.4	0	4.6
Unrealized gains (losses) on derivatives	16.0	(25.9)	(5.6)
Non-cash net periodic benefit (income) costs	(11.5)	(5.7)	5.1
Fabricated Products
Segment Reporting Information [Line Items]
Net non-cash LIFO (benefits) charges	(4.9)	(7.1)	16.5
Environmental expense	1.1	1.7	13.6
Unrealized gains (losses) on derivatives	15.2	(29.9)	(0.7)
United States
Segment Reporting Information [Line Items]
Number of production facilities	11
Canada
Segment Reporting Information [Line Items]
Number of production facilities	1
Anglesey Aluminium Limited
Segment Reporting Information [Line Items]
Ownership percentage of Anglesey	49.00%
VEBAs
Segment Reporting Information [Line Items]
Non-cash net periodic benefit (income) costs	(11.9)	(6.0)	5.1
Idled equipment \| Fabricated Products
Segment Reporting Information [Line Items]
Impairment charge	4.4		2.0
Sale of Greenwood, South Carolina facility \| Fabricated Products
Segment Reporting Information [Line Items]
Impairment charge			1.9
Sale of Greenwood, South Carolina facility \| Idled equipment \| Fabricated Products
Segment Reporting Information [Line Items]
Impairment charge			$ 3.9

Number of Production Facilities.

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

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The charge against earnings resulting from the aggregate write down of all assets from their carrying value to their fair value.

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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.

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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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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.

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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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