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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | $ 968 | $ 1,114 | $ 1,019
Fair Value, Inputs, Level 1, 2 and 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 588 | 582 |
Fair Value, Inputs, Level 1, 2 and 3 | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 78 | 97 |
Fair Value, Inputs, Level 1, 2 and 3 | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 16 | 37 |
Fair Value, Inputs, Level 1, 2 and 3 | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 12 | 13 |
Fair Value, Inputs, Level 1, 2 and 3 | Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 1 | 1 |
Fair Value, Inputs, Level 1, 2 and 3 | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 200 | 256 |
Fair Value, Inputs, Level 1, 2 and 3 | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 141 | 71 |
Fair Value, Inputs, Level 1, 2 and 3 | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 116 | 84 |
Fair Value, Inputs, Level 1, 2 and 3 | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 24 | 23 |
Quoted Prices In Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 307 | 404 |
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 | 78 | 97 |
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 | 16 | 37 |
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 | 12 | 13 |
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 | 200 | 256 |
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] |  |  |
Plan assets at fair value | 281 | 178 |
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 | 141 | 71 |
Significant Other Observable Inputs (Level 2) | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 116 | 84 |
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 | 24 | 23 |
Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | 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 |
NAV |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan assets at fair value | $ 380 | $ 532 |

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