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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

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