Segment and Geographical Area Information, Financial Information by Reporting Segment Table (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Net Sales:
Net sales	$ 314.0		$ 335.5	$ 345.2	$ 365.4	$ 317.6		$ 322.3	$ 338.8	$ 322.6	$ 1,360.1		$ 1,301.3		$ 1,079.1
Segment Operating Income (Loss):
Total operating income (loss)	23.9		56.2	39.6	46.2	15.6		4.9	14.7	19.8	165.9		55.0		41.1
Interest expense											(29.1)		(18.0)		(11.8)
Other income (expense), net											2.8		4.3		(4.2)
Income before income taxes											139.6		41.3		25.1
Depreciation and Amortization:
Depreciation and amortization											26.5		25.2		19.8
Capital expenditures:
Capital expenditures											44.1		32.5		38.9
Segment assets:
Total	1,752.5					1,320.6					1,752.5		1,320.6		1,318.9
Fabricated Products
Net Sales:
Net sales											1,360.1		1,301.3		1,078.8
Segment Operating Income (Loss):
Total operating income (loss)											190.8	[1],[2]	83.6	[1],[2]	81.7	[1],[2]
Depreciation and Amortization:
Depreciation and amortization											26.0		24.8		19.4
Capital expenditures:
Capital expenditures											43.8		32.1		38.0
Segment assets:
Total	771.2					637.0					771.2		637.0		525.3
All Other
Net Sales:
Net sales											0		0		0.3	[3]
Segment Operating Income (Loss):
Total operating income (loss)											(24.9)	[4]	(28.6)	[4]	(40.6)	[4]
Depreciation and Amortization:
Depreciation and amortization											0.5		0.4		0.4
Capital expenditures:
Capital expenditures											0.3		0.4		0.9
Segment assets:
Total	$ 981.3	[5]				$ 683.6	[5]				$ 981.3	[5]	$ 683.6	[5]	$ 793.6	[5]

[1] Fabricated Products segment results for 2012, 2011 and 2010 include non-cash mark-to-market gains (losses) on primary aluminum, natural gas, electricity and foreign currency hedging activities totaling $15.2, $(29.9) and $(0.7), respectively. For further discussion regarding mark-to-market matters, see Note 12.

[2] Operating results in the Fabricated Products segment for 2012, 2011 and 2010 included non-cash LIFO inventory (benefits) charges of $(4.9), $(7.1) and $16.5, respectively. Also included in the Fabricated Products segment operating results for 2012, 2011 and 2010 were $1.1, $1.7 and $13.6, respectively, of environmental expense. Fabricated Products segment operating results for 2012 and 2010 also included $4.4 and $3.9 of asset impairment charge relating to certain Property, plant and equipment, respectively.

[3] Net sales in All Other in 2010 represent residual activity involving primary aluminum purchased by the Company from Anglesey while it continued its smelting operations, prior to September 30, 2009, and resold by the Company in the first quarter of 2010. In connection with Anglesey’s remelt operations beginning in the fourth quarter of 2009, the Company changed its basis of revenue recognition from gross to a net basis (see Note 1).

[4] Operating results in All Other represent operating expenses in the Corporate and Other business unit. Operating results of All Other include VEBA net periodic pension benefit (income) costs of $(11.9), $(6.0) and $5.1 for 2012, 2011 and 2010, respectively.

[5] Assets in All Other represent primarily all of the Company’s cash and cash equivalents, short-term investments, metal and financial derivative assets, net assets in respect of VEBA(s) and net deferred income tax assets.