Financial Instruments and Risk Management - General (Details) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) approach $ / shares shares	Dec. 31, 2019 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	$ 54	$ 35
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	$ 1,400
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months	(2)
Gain (loss) on derivatives included in AOCI, net of tax	$ 3
Commodity contracts | Maximum
Financial Instruments and Risk Management
Period within which derivative will expire	4 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	$ 3,400
Period within which derivative will expire	4 years
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months	$ 18
Gain (loss) on derivatives included in AOCI, net of tax	37
Cross-currency swap
Financial Instruments and Risk Management
Notional amount of derivatives			$ 1,000
Gain (loss) on derivatives included in AOCI, net of tax	$ (1)
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	2.8