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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, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | $ 6,652 | $ 5,959 | $ 5,225
Cash and cash equivalents |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 182 | 81 |
Equity investments, Pooled funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 4 | 119 |
Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | (1,424) | (1,205) |
Corporate bonds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 4 |  |
Government and agency bonds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 2,872 | 2,667 |
Annuities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 2,625 | 1,849 |
Fixed Income Investments, Pooled Funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 875 | 1,486 |
Real estate |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 117 | 180 |
Other Investments, Pooled Funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 1,397 | 782 |
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 | 3,062 | 2,748 |
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 | 182 | 81 |
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 | 4 |  |
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 | 2,872 | 2,667 |
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) | Other Investments, Pooled Funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 4 | 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 | (1,424) | (1,205) |
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 | (1,424) | (1,205) |
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 |  |  |
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) | Other 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) |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | 2,625 | 1,849 | $ 1,688
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 |  |  |
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 | 2,625 | 1,849 |
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) | Other Investments, Pooled Funds |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Defined benefit plan, plan assets | $ 0 | $ 0 |

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