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Employee Benefits - Schedule of Fair Value of U.S Plan Assets (Details) - Pension Plan - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
U.K. |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | $ 5,906 | $ 5,675 | $ 5,903
U.K. | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 209 | 86 |
U.K. | Equity derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | (771) | 10 |
U.K. | Alternative investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 911 | 773 |
U.K. | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 2,571 | 1,947 |
U.K. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 209 | 86 |
U.K. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.K. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.K. | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | (346) | 413 |
U.K. | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.K. | Significant Other Observable Inputs (Level 2) | Equity derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | (771) | 10 |
U.K. | Significant Other Observable Inputs (Level 2) | Alternative investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.K. | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 1,909 | 1,773 |
U.K. | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.K. | Significant Unobservable Inputs (Level 3) | Equity derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.K. | Significant Unobservable Inputs (Level 3) | Alternative investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 1,958 | 1,683 | $ 1,951
U.S. | Fixed income derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  | 65 |
U.S. | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 56 | 100 |
U.S. | Large cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 313 | 268 |
U.S. | Small cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 17 | 15 |
U.S. | Large cap international |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 90 | 64 |
U.S. | Equity derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 111 | 81 |
U.S. | International pooled funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 270 | 196 |
U.S. | Pooled funds large cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 12 |  |
U.S. | Pooled funds small cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 114 | 52 |
U.S. | Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 110 | 105 |
U.S. | Government and agency bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 148 | 132 |
U.S. | Corporate Bonds And Pooled Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 290 | 255 |
U.S. | Commodity Derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  | 22 |
U.S. | Real estate and REITS |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 82 | 61 |
U.S. | Alternative investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 345 | 267 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 672 | 727 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  | 65 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 56 | 100 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 313 | 268 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Small cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 17 | 15 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap international |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 90 | 64 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 78 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | International pooled funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds large cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 |  |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds small cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Government and agency bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 114 | 76 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate Bonds And Pooled Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodity Derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  | 0 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate and REITS |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 82 | 61 |
U.S. | Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 255 | 186 |
U.S. | Significant Other Observable Inputs (Level 2) | Fixed income derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  | 0 |
U.S. | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Other Observable Inputs (Level 2) | Large cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Other Observable Inputs (Level 2) | Small cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Other Observable Inputs (Level 2) | Large cap international |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Other Observable Inputs (Level 2) | Equity derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 111 | 3 |
U.S. | Significant Other Observable Inputs (Level 2) | International pooled funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Other Observable Inputs (Level 2) | Pooled funds large cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 |  |
U.S. | Significant Other Observable Inputs (Level 2) | Pooled funds small cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Other Observable Inputs (Level 2) | Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 110 | 105 |
U.S. | Significant Other Observable Inputs (Level 2) | Government and agency bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 34 | 56 |
U.S. | Significant Other Observable Inputs (Level 2) | Corporate Bonds And Pooled Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Other Observable Inputs (Level 2) | Commodity Derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  | 22 |
U.S. | Significant Other Observable Inputs (Level 2) | Real estate and REITS |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Other Observable Inputs (Level 2) | Alternative investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Fixed income derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Large cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Small cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Large cap international |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Equity derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | International pooled funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Pooled funds large cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 |  |
U.S. | Significant Unobservable Inputs (Level 3) | Pooled funds small cap domestic |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Government and agency bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Corporate Bonds And Pooled Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Commodity Derivatives |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Real estate and REITS |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | 0 | 0 |
U.S. | Significant Unobservable Inputs (Level 3) | Alternative investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total | $ 0 | $ 0 |

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