Segment and Geographical Area Information, Textuals (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Decrease in Fabricated Products segment operating income relating to primary aluminum hedging activities		$ 15.5		$ 21.1
Increase in Fabricated Products segment asset related to primary aluminum hedging activities					0.6
Net non-cash LIFO (benefits) charges	(5.7)	(7.1)	(13.5)	12.8
Unrealized gains (losses) on derivatives	12.2	(12.7)	16.4	(19.8)
Fabricated Products
Segment Reporting Information [Line Items]
Unrealized gains (losses) on derivatives	$ 12.3	$ (16.8)	$ 15.3	$ (22.0)
United States
Segment Reporting Information [Line Items]
Number of production facilities			11
Canada
Segment Reporting Information [Line Items]
Number of production facilities			1