Segment Information, Textuals (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended					3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Fabricated Products	Jun. 30, 2012 Fabricated Products	Jun. 30, 2013 Fabricated Products	Jun. 30, 2012 Fabricated Products	Jun. 30, 2013 United States productionfacilities	Jun. 30, 2013 Canada productionfacilities	Dec. 31, 2012 Anglesey Aluminium Limited	Jun. 30, 2013 VEBAs	Jun. 30, 2012 VEBAs	Jun. 30, 2013 VEBAs	Jun. 30, 2012 VEBAs
Segment Reporting Information [Line Items]
Number of production facilities									11	1
Ownership percentage of Anglesey											49.00%
Net non-cash LIFO benefits			$ 12.9	$ 7.8	$ 9.2	$ 4.9	$ 12.9	$ 7.8
Unrealized (losses) gains on derivatives	(5.1)	0.6	(5.4)	4.2	(4.2)	(0.1)	(4.9)	3.0
VEBA net periodic pension benefit income			$ 11.3	$ 6.0								$ 5.7	$ 3.0	$ 11.3	$ 6.0