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Pension and Other Postretirement Plans - Pension and Other Postretirement Plan Assets (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Level 3 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 401 | $ 409 | $ 350
Pension Benefits |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 4,069 | 4,964 | 3,827
Pension Benefits | Level 1 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 187 | 513 |
Pension Benefits | Level 2 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 681 | 825 |
Pension Benefits | Level 3 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 350 | 357 |
Pension Benefits | Assets Measured at NAV |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 2,851 | 3,269 |
Pension Benefits | Equity securities funds |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 1,606 | 1,957 |
Pension Benefits | Equity securities funds | Level 1 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 55 | 47 |
Pension Benefits | Equity securities funds | Level 2 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 125 | 117 |
Pension Benefits | Equity securities funds | Level 3 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 96 | 71 |
Pension Benefits | Equity securities funds | Assets Measured at NAV |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 1,330 | 1,722 |
Pension Benefits | Fixed-income securities |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 1,644 | 1,732 |
Pension Benefits | Fixed-income securities | Level 1 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Pension Benefits | Fixed-income securities | Level 2 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 548 | 687 |
Pension Benefits | Fixed-income securities | Level 3 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 49 | 69 |
Pension Benefits | Fixed-income securities | Assets Measured at NAV |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 1,047 | 976 |
Pension Benefits | Alternatives |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 669 | 776 |
Pension Benefits | Alternatives | Level 1 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Pension Benefits | Alternatives | Level 2 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Pension Benefits | Alternatives | Level 3 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 195 | 205 |
Pension Benefits | Alternatives | Assets Measured at NAV |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 474 | 571 |
Pension Benefits | Other investments |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 150 | 499 |
Pension Benefits | Other investments | Level 1 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 132 | 466 |
Pension Benefits | Other investments | Level 2 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 8 | 21 |
Pension Benefits | Other investments | Level 3 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 10 | 12 |
Pension Benefits | Other investments | Assets Measured at NAV |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Other Postretirement Benefits |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 51 | 52 | $ 53
Other Postretirement Benefits | Deposit administration fund |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 51 | 52 |
Other Postretirement Benefits | Deposit administration fund | Level 1 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Other Postretirement Benefits | Deposit administration fund | Level 2 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Other Postretirement Benefits | Deposit administration fund | Level 3 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 51 | 52 |
Other Postretirement Benefits | Deposit administration fund | Assets Measured at NAV |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 0 | $ 0 |

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