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Financial Instruments and Risk Management - General (Details)
shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
item
shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Jun. 30, 2017
USD ($)
Financial Instruments and Risk Management        
Costs associated with terminated interest rate contracts   $ 20.0 $ 16.0  
Aggregate fair value of derivative instruments with credit-risk-related contingent features that were in a net liability position $ 27.0 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    
Cash Flow Hedging        
Financial Instruments and Risk Management        
Period within which derivative will expire 2 years      
Intercompany Debt Foreign Currency Denominated        
Financial Instruments and Risk Management        
Gain (loss) on derivatives included in accumulated other comprehensive earnings (loss), net of tax $ 27.0      
Net gain expected to be recognized in the consolidated statement of earnings during the next 12 months $ 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 | item 2      
Notional amount of derivatives $ 581.0      
Gain (loss) on derivatives included in accumulated other comprehensive earnings (loss), net of tax 36.0      
Net gain expected to be recognized in the consolidated statement of earnings during the next 12 months 32.0      
Commodity contracts | Derivatives Designated As Hedging Instruments | Cash Flow Hedging        
Financial Instruments and Risk Management        
Notional amount of derivatives 506.0      
Interest rate swap and option contracts        
Financial Instruments and Risk Management        
Notional amount of derivatives $ 1,700.0      
Period within which derivative will expire 2 years      
Currency Exchange Rate Risk        
Financial Instruments and Risk Management        
Notional amount of derivatives $ 2.3      
Gain (loss) on derivatives included in accumulated other comprehensive earnings (loss), net of tax $ 4.0      
Total Return Swap [Member]        
Financial Instruments and Risk Management        
Combined notional value (in shares) | shares 2.7      
Fair value of swaps $ 5.0      
Cross-currency swap        
Financial Instruments and Risk Management        
Notional amount of derivatives       $ 1,000.0
Period within which derivative will expire 3 years      
Net gain expected to be recognized in the consolidated statement of earnings during the next 12 months $ 117.0