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Financial Instruments and Risk Management - General (Details)
$ / shares in Units, item in Millions, $ in Millions
9 Months Ended
Sep. 30, 2017
USD ($)
item
approach
Sep. 30, 2017
USD ($)
item
approach
$ / shares
Sep. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Financial Instruments and Risk Management        
Costs associated with terminated interest rate contracts     $ 30.0  
Aggregate fair value of derivative instruments with credit-risk-related contingent features that were in a net liability position $ 15.0 $ 15.0   $ 44.0
Collateral amount posted for derivative instruments with credit-risk-related contingent features that were in a net liability position $ 0.0 $ 0.0   $ 0.0
Commodity contracts        
Financial Instruments and Risk Management        
Number of methods through which entity manages commodity price risk in connection with market price fluctuations of aluminum ingot | approach 2 2    
Gain (loss) on derivatives included in accumulated other comprehensive earnings (loss), net of tax $ 24.0 $ 24.0    
Net gain expected to be recognized in the consolidated statement of earnings during the next 12 months   19.0    
Commodity contracts | Derivatives Designated As Hedging Instruments        
Financial Instruments and Risk Management        
Notional amount of derivatives 732.0 $ 732.0    
Commodity contracts | Maximum        
Financial Instruments and Risk Management        
Period within which derivative will expire   3 years    
Interest rate swap agreements        
Financial Instruments and Risk Management        
Notional amount of derivatives 1.0 $ 1.0    
Currency Exchange Rate Risk        
Financial Instruments and Risk Management        
Notional amount of derivatives 2,000.0 $ 2,000.0    
Period within which derivative will expire   2 years    
Net gain expected to be recognized in the consolidated statement of earnings during the next 12 months 2.0      
Net after-tax gain (loss) included in AOCI 3.0      
Cross-currency swap        
Financial Instruments and Risk Management        
Gain (loss) on derivatives included in accumulated other comprehensive earnings (loss), net of tax (22.0) $ (22.0)    
Amount of hedge contract     $ 1,000.0  
Fair value of swaps $ (96.0) $ (96.0)    
Equity contracts        
Financial Instruments and Risk Management        
Change in company's stock price (in dollars per share) | $ / shares   $ 1    
Notional value of the swap (in shares) | item 2.7 2.7    
Fair value of swaps $ 2.0 $ 2.0