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Financial Instruments and Risk Management - General (Details)
$ / shares in Units, shares in Millions, $ in Millions
9 Months Ended
Sep. 30, 2019
USD ($)
approach
$ / shares
shares
Dec. 31, 2018
USD ($)
Dec. 31, 2016
USD ($)
Financial Instruments and Risk Management      
Aggregate fair value of derivative instruments with credit-risk-related contingent features that were in a net liability position $ 32 $ 46  
Collateral amount posted for derivative instruments with credit-risk-related contingent features that were in a net liability position $ 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 $ 1,300    
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months (21)    
Gain (loss) on derivatives included in AOCI, net of tax (24)    
Commodity contracts | Derivatives Designated As Hedging Instruments | Cash Flow Hedging      
Financial Instruments and Risk Management      
Notional amount of derivatives $ 1,200    
Commodity contracts | Maximum      
Financial Instruments and Risk Management      
Period within which derivative will expire 3 years    
Interest rate swap and option contracts      
Financial Instruments and Risk Management      
Notional amount of derivatives $ 1,300    
Interest rate swap and option contracts | Maximum      
Financial Instruments and Risk Management      
Period within which derivative will expire 2 years    
Currency Exchange Rate Risk      
Financial Instruments and Risk Management      
Notional amount of derivatives $ 3,800    
Period within which derivative will expire 5 years    
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months $ 3    
Gain (loss) on derivatives included in AOCI, net of tax 10    
Net after-tax gain (loss) included in AOCI (10)    
Cross-currency swap      
Financial Instruments and Risk Management      
Notional amount of derivatives 250   $ 1,000
Gain (loss) on derivatives included in AOCI, net of tax (6)    
Notional amount terminated $ 750    
Equity contracts      
Financial Instruments and Risk Management      
Change in company's stock price (in dollars per share) | $ / shares $ 1    
Combined notional value (in shares) | shares 2.9