Derivative Financial Instruments and Related Hedging Programs, Hedging Discussion (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 mmlbs	Mar. 31, 2014 mmlbs	Dec. 31, 2014
Derivative Financial Instruments and Related Hedging Programs (Textuals) [Abstract]
Aggregate fair value of derivative instruments that contain credit-risk-related contingency features that were in a net liability position	$ 13.7		$ 11.4
Total fabricated products shipments containing fixed price terms	41.1	34.4
Remainder of 2015
Derivative [Line Items]
Percentage of Electricity Purchases For Which Company Exposure To Fluctuations In Electricity Prices Have Been Reduced	55.00%
Derivative Financial Instruments and Related Hedging Programs (Textuals) [Abstract]
Firm price customer sales contracts containing price risk on anticipated purchases of primary aluminum	96.3
2016
Derivative [Line Items]
Percentage of Electricity Purchases For Which Company Exposure To Fluctuations In Electricity Prices Have Been Reduced	54.00%
Derivative Financial Instruments and Related Hedging Programs (Textuals) [Abstract]
Firm price customer sales contracts containing price risk on anticipated purchases of primary aluminum	1.9
2017
Derivative [Line Items]
Percentage of Electricity Purchases For Which Company Exposure To Fluctuations In Electricity Prices Have Been Reduced	27.00%