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Employee Benefits - Schedule of Fair Value of Other Plan Assets (Details) - Other - Pension Plan - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | $ 1,256 | $ 1,076 | $ 1,019
Cash and cash equivalents |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 11 | 11 |
Pooled funds - Global |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 370 | 322 |
Pooled funds small cap domestic |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 26 | 36 |
Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total |  | 20 |
Fixed income securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 211 | 166 |
Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 40 | 37 |
Pooled funds - Fixed income securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 566 | 469 |
Alternative investments |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 26 | 9 |
Pooled Funds, Real Estate |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 6 | 6 |
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 11 | 11 |
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] |  |  |
Total | 11 | 11 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds - Global |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds small cap domestic |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 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] |  |  |
Total |  | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 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] |  |  |
Total | 0 | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled funds - Fixed income securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 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] |  |  |
Total | 0 | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pooled Funds, Real Estate |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 251 | 203 |
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Pooled funds - Global |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Pooled funds small cap domestic |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total |  | 0 |
Significant Other Observable Inputs (Level 2) | Fixed income securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 211 | 166 |
Significant Other Observable Inputs (Level 2) | Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 40 | 37 |
Significant Other Observable Inputs (Level 2) | Pooled funds - Fixed income securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Alternative investments |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Other Observable Inputs (Level 2) | Pooled Funds, Real Estate |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Pooled funds - Global |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Pooled funds small cap domestic |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total |  | 0 |
Significant Unobservable Inputs (Level 3) | Fixed income securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Derivatives |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Pooled funds - Fixed income securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Alternative investments |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | 0 | 0 |
Significant Unobservable Inputs (Level 3) | Pooled Funds, Real Estate |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Total | $ 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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