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Retirement Plans and Other Postretirement Benefits - Narrative (Details) - USD ($) |  | 3 Months Ended |  |
--- | --- | --- | --- | ---
Jan. 04, 2021 | Mar. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Initial pre-65 ultimate health care cost trend rate (as a percent) |  |  |  | 4.75%
Initial post-65 healthcare cost trend rate (as a percent) |  |  | 2.00% |
Transfer to active union medical account | $ (106,000,000) |  |  |
Contributions |  |  |  |
Expected voluntary employer contributions in the next fiscal year |  | $ 0 |  |
Expected voluntary employer contributions in the year after next fiscal year |  | 0 |  |
Other Benefits |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Transfer to active union medical account | 106,000,000 |  |  |
Contributions |  |  |  |
Estimated future employer contributions in next three years |  | 0 |  |
Pension Benefits |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Transfer to active union medical account | $ (106,000,000) |  |  |
Contributions |  |  |  |
Voluntary contributions |  | 0 |  |
Minimum employer contributions for the next three years |  | 0 |  |
Expected voluntary employer contributions in the remainder of the current fiscal year |  | $ 100,000,000 |  |

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