Financial Instruments and Risk Management - General (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 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	$ 83	$ 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	$ 1,800
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months	(34)
Gain (loss) on derivatives included in AOCI, net of tax	(42)
Commodity contracts | Derivatives Designated As Hedging Instruments | Cash Flow Hedging
Financial Instruments and Risk Management
Notional amount of derivatives	$ 1,600
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	$ 968,000
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months	$ (1)
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	$ 2,800
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	$ 13
Gain (loss) on derivatives included in AOCI, net of tax	38
Cross-currency swap
Financial Instruments and Risk Management
Notional amount of derivatives			$ 1,000
Gain (loss) on derivatives included in AOCI, net of tax	$ (2)
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