Pension and Other Postretirement Benefits - Additional Information (Details) $ in Millions			1 Months Ended	3 Months Ended		12 Months Ended
	Jul. 25, 2019 employee	Apr. 13, 2018 employee	Jun. 30, 2019 employee	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Decrease in pension liability, plan amendment						$ 257	$ 268	$ 298
Offset to AOCI, net of tax						(46)	(388)	255
Curtailment (charge) benefit								(9)
Duration of labor agreement		5 years
Number of employees included in labor agreement | employee	560	1,300	3,400
Employees subject to retirement benefit increase under labor agreement | employee		300
Labor agreement, one time signing bonus							9
Labor agreement, number of years covered	4 years		3 years
Restructuring and other charges						182	582	163
Transfer from Engineered Structures to Discontinued Operations (Arconic Corporation)
Defined Benefit Plan Disclosure [Line Items]
Decrease in pension liability, plan amendment							(42)
Restructuring Charges
Defined Benefit Plan Disclosure [Line Items]
Curtailment (charge) benefit							16
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Curtailment (charge) benefit						0	0	(23)
Restructuring and other charges						69
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Decrease in pension liability, plan amendment						6		32
Offset to AOCI, net of tax				$ 13				4
Curtailment (charge) benefit					$ (9)	2	58	28
United States | Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Decrease in pension liability, plan amendment							75	136
Offset to AOCI, net of tax								141
Curtailment (charge) benefit							(17)	(5)
United States | Other postretirement benefits | Restructuring Charges
Defined Benefit Plan Disclosure [Line Items]
Curtailment (charge) benefit						66
Pension Settlement Cost
Defined Benefit Plan Disclosure [Line Items]
Restructuring and other charges						$ 8	$ 9	$ 96