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Financial Instruments and Risk Management - General (Details)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
approach
$ / shares
Dec. 31, 2017
USD ($)
Sep. 30, 2016
USD ($)
Financial Instruments and Risk Management      
Aggregate fair value of derivative instruments with credit-risk-related contingent features that were in a net liability position $ 20.0 $ 27.0  
Collateral amount posted for derivative instruments with credit-risk-related contingent features that were in a net liability position $ 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    
Notional amount of derivatives $ 844.0    
Net gain expected to be recognized in the consolidated statement of earnings during the next 12 months 14.0    
Gain (loss) on derivatives included in AOCI, net of tax 10.0    
Commodity contracts | Derivatives Designated As Hedging Instruments | Cash Flow Hedging      
Financial Instruments and Risk Management      
Notional amount of derivatives $ 822.0    
Commodity contracts | Maximum      
Financial Instruments and Risk Management      
Period within which derivative will expire 2 years    
Interest rate swap and option contracts      
Financial Instruments and Risk Management      
Notional amount of derivatives $ 1,300.0    
Period within which derivative will expire 2 years    
Currency Exchange Rate Risk      
Financial Instruments and Risk Management      
Notional amount of derivatives $ 2,200.0    
Net after-tax gain (loss) included in AOCI 3.0    
Cross-currency swap      
Financial Instruments and Risk Management      
Gain (loss) on derivatives included in AOCI, net of tax (43.0)    
Amount of hedge contract     $ 1,000.0
Fair value of swaps $ (164.0)    
Equity contracts      
Financial Instruments and Risk Management      
Change in company's stock price (in dollars per share) | $ / shares $ 1    
Fair value of swaps $ 2.0