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Pension and Other Postretirement Benefit Plans - Fair Values of Other Postretirement Benefit Plan Assets (Details) - Other Postretirement Benefits - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net assets in fair value hierarchy | $ 582 | $ 540 |
Investments measured at NAV practical expedient | 532 | 479 |
Plan assets at fair value | 1,114 | 1,019 | $ 1,913
Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 97 | 125 |
Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 37 | 48 |
Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 13 | 10 |
Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 1 | 1 |
Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 256 | 231 |
Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 71 | 43 |
Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 84 | 60 |
Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 23 | 22 |
Quoted Prices In Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net assets in fair value hierarchy | 404 | 415 |
Quoted Prices In Active Markets for Identical Assets (Level 1) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 97 | 125 |
Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 37 | 48 |
Quoted Prices In Active Markets for Identical Assets (Level 1) | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 13 | 10 |
Quoted Prices In Active Markets for Identical Assets (Level 1) | Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 1 | 1 |
Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 256 | 231 |
Quoted Prices In Active Markets for Identical Assets (Level 1) | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Quoted Prices In Active Markets for Identical Assets (Level 1) | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Quoted Prices In Active Markets for Identical Assets (Level 1) | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net assets in fair value hierarchy | 178 | 125 |
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 71 | 43 |
Significant Other Observable Inputs (Level 2) | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 84 | 60 |
Significant Other Observable Inputs (Level 2) | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 23 | 22 |
Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net assets in fair value hierarchy | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | $ 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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