Segment and Geographical Area Information Segment and Geographical Area Information, Information by Geographical Area (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 Suppliers		Dec. 31, 2011		Dec. 31, 2010
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets [LIne Items]
Net sales to unaffiliated customers	$ 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
Income taxes paid											1.8		3.5		0.8
Long-lived assets	384.3	[1]				367.8	[1]				384.3	[1]	367.8	[1]	354.1	[1]
Percentage of total revenue from the Company's largest customer											22.00%		21.00%		23.00%
Export sales percentage											18.00%		14.00%		13.00%
Number of Company's major suppliers											5
Top five major suppliers
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets [LIne Items]
Percentage of primary aluminum from Company's supplier over total purchases											78.00%		83.00%		85.00%
Largest supplier
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets [LIne Items]
Percentage of primary aluminum from Company's supplier over total purchases											29.00%		32.00%		33.00%
Second and third largest suppliers
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets [LIne Items]
Percentage of primary aluminum from Company's supplier over total purchases											31.00%		34.00%		36.00%
Fabricated Products
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets [LIne Items]
Net sales to unaffiliated customers											1,360.1		1,301.3		1,078.8
Long-lived assets	380.0	[1]				363.3	[1]				380.0	[1]	363.3	[1]	348.8	[1]
Fabricated Products | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets [LIne Items]
Net sales to unaffiliated customers											1,256.5		1,195.1		991.2
Income taxes paid											0.5		1.7		0.1
Long-lived assets	367.5	[1]				351.4	[1]				367.5	[1]	351.4	[1]	337.3	[1]
Fabricated Products | Canada
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets [LIne Items]
Net sales to unaffiliated customers											103.6		106.2		87.6
Income taxes paid											1.3		1.8		0.7
Long-lived assets	12.5	[1]				11.9	[1]				12.5	[1]	11.9	[1]	11.5	[1]
All Other
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets [LIne Items]
Net sales to unaffiliated customers											0		0		0.3	[2]
Long-lived assets	4.3	[1]				4.5	[1]				4.3	[1]	4.5	[1]	5.3	[1]
All Other | United States
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets [LIne Items]
Long-lived assets	4.3	[1]				4.5	[1]				4.3	[1]	4.5	[1]	5.3	[1]
All Other | United Kingdom
Revenues from External Customers, Income Taxes Paid, and Long-Lived Assets [LIne Items]
Net sales to unaffiliated customers											$ 0		$ 0		$ 0.3

[1] Long-lived assets represent Property, plant and equipment, net.

[2] 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).