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Pension and Other Postretirement Benefits - Additional Information (Details) $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Apr. 13, 2018 employee | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Curtailments |  |  |  | $ 9 |  |  |  |
Offset to AOCI, net of tax |  |  |  |  |  | $ 255 | $ (220) | $ (482)
Duration of labor agreement | 5 years |  |  |  |  |  |  |
Number of employees included in Labor Agreement | employee | 1,300 |  |  |  |  |  |  |
Employees subject to retirement benefit increase under Labor Agreement | employee | 300 |  |  |  |  |  |  |
Restructuring and other charges |  |  |  |  |  | 9 | 165 | 155
Pension benefits |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Curtailments |  |  |  |  |  | 23 | 0 | 0
Service cost |  |  |  |  |  | 46 | 90 | 155
Other postretirement benefits |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Decrease in pension liability, plan amendment |  |  | $ 32 |  |  |  |  |
Curtailments |  | $ 9 | (28) |  |  | (28) | 0 | 0
Offset to AOCI, net of tax |  | 13 | $ 4 |  |  |  |  |
Service cost |  |  |  |  |  | 7 | $ 7 | 13
United States | Pension benefits |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Decrease in pension liability, plan amendment |  |  |  |  | $ 136 |  |  |
Curtailments |  |  |  |  | 5 |  |  |
Offset to AOCI, net of tax |  |  |  |  | $ 141 |  |  |
Pension Settlement Cost |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Restructuring and other charges |  | $ 96 |  |  |  | $ 96 |  | $ 2

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