Restructuring and Other Charges - Narrative (Details) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 18, 2019 USD ($)	Sep. 30, 2019 USD ($) position	Sep. 30, 2018 USD ($) position	Sep. 30, 2019 USD ($) position	Sep. 30, 2018 USD ($) position	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)		$ 119	$ (2)	$ 630	$ 20
Restructuring and other charges (benefits) after tax		114	(3)	521	14
Revenue from Contract with Customer, Excluding Assessed Tax		3,557	3,522	10,787	10,551
Cash payments for restructuring				65				$ 49
Asset Impairment associated with the sale of business
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)		102
Restructuring and other charges (benefits) after tax		102
Sale Of Company's Brazilian Rolling Mill Operations
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)		59
UK Forgings
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)		43
Impairment Of Property, Plant, And Equipment
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)		8
Restructuring and other charges (benefits) after tax		6
Pension Settlement Cost
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)		4	4	8	4
Restructuring and other charges (benefits) after tax		3	3	6	3
Restructuring, Accelerated Depreciation
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)		4		6
Restructuring and other charges (benefits) after tax		3		5
Restructuring Programs Layoffs
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)		2	8	97	16
Restructuring and other charges (benefits) after tax		$ 1	$ 6	$ 74	$ 12
Number of employees associated with layoff costs | position		56	85	1,183	125
Layoff costs
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)		$ (1)	$ (1)	$ (4)
Restructuring and other charges (benefits) after tax		$ (1)	(1)	(4)
Cash payments for restructuring				61				47
Impairment Of Disks
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)				428
Restructuring and other charges (benefits) after tax				345
Impairment for a trade name intangible asset and fixed assets
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)				24
Restructuring and other charges (benefits) after tax				18
Other exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)				12
Restructuring and other charges (benefits) after tax				9
Cash payments for restructuring				4				$ 2
Loss On Sale Related To Small Additive Business
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)				12
Restructuring and other charges (benefits) after tax				9
Executive Severance Net Of The Benefit Of Forfeited Executive Stock Compensation
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)				2
Restructuring and other charges (benefits) after tax				1
Other Miscellaneous Charges
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)			3	1	$ 7
Restructuring and other charges (benefits) after tax			2		5
Elimination Of Life Insurance Benefits
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)				(58)
Restructuring and other charges (benefits) after tax				$ (45)
Postretirement Curtailment Benefit
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)			28		28
Restructuring and other charges (benefits) after tax			$ (22)		22
Contract Termination Cost and Asset Impairments
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)					12
Restructuring and other charges (benefits) after tax					9
Pension Curtailment Charge
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)					14
Restructuring and other charges (benefits) after tax					11
New York
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)					5
Restructuring and other charges (benefits) after tax					4
Other Adjustments
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)					10
Restructuring and other charges (benefits) after tax					$ 8
Restructuring Programs Layoffs 2019
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | position				1,183
Approximate number of employees already laid off | position		745		745
Cash payments for restructuring		$ 21		$ 61
Corporate Segment | Restructuring Programs Layoffs
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | position			20	463	36
Engineered Products and Forgings
Restructuring Cost and Reserve [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1,794	$ 1,683	$ 5,372	$ 5,083
Engineered Products and Forgings | Restructuring Programs Layoffs
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | position			65	350	89
Global Rolled Products
Restructuring Cost and Reserve [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		$ 1,763	$ 1,839	$ 5,415	$ 5,468
Global Rolled Products | Restructuring Programs Layoffs
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | position				370
Subsequent Event | GRP Operations Curtailment Benefit
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges (benefits)	$ 4
Scenario, Forecast | Layoff costs
Restructuring Cost and Reserve [Line Items]
Cash payments for restructuring						$ 15
Scenario, Forecast | GRP Operations Curtailment Benefit
Restructuring Cost and Reserve [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax							$ 50