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Pension and Other Postretirement Plans - Pension and Other Postretirement Plan Assets (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
Level 3 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | $ 350 | $ 383 | $ 287 |
Pension Benefits |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 3,827 | 3,932 |  | $ 3,355
Pension Benefits | Level 1 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 478 | 301 |  |
Pension Benefits | Level 2 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 751 | 1,091 |  |
Pension Benefits | Level 3 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 297 | 329 |  |
Pension Benefits | Assets Measured at NAV |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 2,301 | 2,211 |  |
Pension Benefits | Equity securities funds |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 1,394 | 1,406 |  |
Pension Benefits | Equity securities funds | Level 1 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 254 | 269 |  |
Pension Benefits | Equity securities funds | Level 2 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 106 | 133 |  |
Pension Benefits | Equity securities funds | Level 3 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 0 | 0 |  |
Pension Benefits | Equity securities funds | Assets Measured at NAV |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 1,034 | 1,004 |  |
Pension Benefits | Fixed-income securities |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 1,431 | 1,470 |  |
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 | 605 | 834 |  |
Pension Benefits | Fixed-income securities | Level 3 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 21 | 18 |  |
Pension Benefits | Fixed-income securities | Assets Measured at NAV |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 805 | 618 |  |
Pension Benefits | Alternatives |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 596 | 637 |  |
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 | 134 | 139 |  |
Pension Benefits | Alternatives | Assets Measured at NAV |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 462 | 498 |  |
Pension Benefits | Other |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 406 | 419 |  |
Pension Benefits | Other | Level 1 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 224 | 32 |  |
Pension Benefits | Other | Level 2 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 40 | 124 |  |
Pension Benefits | Other | Level 3 |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 142 | 172 |  |
Pension Benefits | Other | Assets Measured at NAV |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 0 | 91 |  |
Other Postretirement Benefits |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 53 | 54 |  | $ 55
Other Postretirement Benefits | Deposit administration fund |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets | 53 | 54 |  |
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 | 53 | 54 |  |
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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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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