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Employee Benefits - Schedule of Fair Value of U.K. Plan Assets (Details) - Pension Plan - U.K. - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | $ 5,959 | $ 5,225 | $ 5,906
Cash and cash equivalents |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 81 | 96 |
Equity investments, Pooled funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 119 | 212 |
Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | (1,205) | (949) |
Corporate bonds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets |  | 367 |
Government and agency bonds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 2,667 | 2,079 |
Annuities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 1,849 | 1,688 |
Fixed Income Investments, Pooled Funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 1,486 | 889 |
Real estate |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 180 | 149 |
Alternative investments |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 782 | 694 |
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 2,748 | 2,175 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 81 | 96 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity investments, Pooled funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets |  | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Government and agency bonds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 2,667 | 2,079 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Annuities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed Income Investments, Pooled Funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | (1,205) | (582) |
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Equity investments, Pooled funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | (1,205) | (949) |
Significant Other Observable Inputs (Level 2) | Corporate bonds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets |  | 367 |
Significant Other Observable Inputs (Level 2) | Government and agency bonds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Annuities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Fixed Income Investments, Pooled Funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Real estate |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Alternative investments |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 1,849 | 1,688 | $ 1,909
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Equity investments, Pooled funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Corporate bonds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets |  | 0 |
Significant Unobservable Inputs (Level 3) | Government and agency bonds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Annuities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 1,849 | 1,688 |
Significant Unobservable Inputs (Level 3) | Fixed Income Investments, Pooled Funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Real estate |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Alternative investments |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | $ 0 | $ 0 |

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