Restructuring and Other Charges - Narrative (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) employee position	Mar. 31, 2019 USD ($) employee	Dec. 31, 2019 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 21	$ 12
Restructuring and other charges (benefits) after tax	23	10
Gain on sale of assets	(2)	(2)
Cash payments for restructuring	7		$ 79
Extrusions Busines
Restructuring Cost and Reserve [Line Items]
Gain (loss) on sale of business	27
Gain on sale of business, net of tax	20
Layoff costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	22	65
Restructuring and other charges (benefits) after tax	17	$ 51
Cash payments for restructuring	$ 7		$ 74
Restructuring Programs Layoffs 2020
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | employee	460
Approximate number of employees already laid off | employee	0
Impairments Of Assets Associated With Agreement To Sell Aerospace Components Business
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 12
Restructuring and other charges (benefits) after tax	12
UK Forgings
Restructuring Cost and Reserve [Line Items]
Restructuring charges	6
Restructuring and other charges (benefits) after tax	6
Sale Of Company's Brazilian Rolling Mill Operations
Restructuring Cost and Reserve [Line Items]
Restructuring charges	6
Restructuring and other charges (benefits) after tax	6
Other Exit Costs Related To Prior Programs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	6
Restructuring and other charges (benefits) after tax	6
Reversal Of Prior Period Programs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	(2)
Restructuring and other charges (benefits) after tax	(2)
Sale Of Assets
Restructuring Cost and Reserve [Line Items]
Gain on sale of assets	2
Gain on sale of assets, net of tax	$ 2
Restructuring Programs Layoffs 2019
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs	1,310	800
Approximate number of employees already laid off | position	1,053
Cash payments for restructuring	$ 7
Executive Severance Net Of The Benefit Of Forfeited Executive Stock Compensation
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 2
Restructuring and other charges (benefits) after tax		1
Pension Settlement Cost
Restructuring Cost and Reserve [Line Items]
Restructuring charges		2
Restructuring and other charges (benefits) after tax		2
Other Miscellaneous Charges
Restructuring Cost and Reserve [Line Items]
Restructuring charges		1
Restructuring and other charges (benefits) after tax		1
Elimination Of Life Insurance Benefits
Restructuring Cost and Reserve [Line Items]
Restructuring charges		(58)
Restructuring and other charges (benefits) after tax		$ (45)
Engineered Products and Forgings | Restructuring Programs Layoffs 2020
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | employee	440
Engineered Products and Forgings | Restructuring Programs Layoffs 2019
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | employee		211
Corporate Segment | Restructuring Programs Layoffs 2020
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | employee	20
Corporate Segment | Restructuring Programs Layoffs 2019
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | employee		425
Global Rolled Products | Restructuring Programs Layoffs 2019
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | employee		164