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Pension and Other Postretirement Benefit Plans - Fair Values of Pension Plan Assets (Details) - Pension Plan - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 488 | $ 435 |
United States |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 10,896 | 9,766 | $ 9,266
Fair Value of Plan Assets Excluding Net Asset Value Investments | 5,189 | 4,542 |
Investments measured at NAV practical expedient | 5,707 | 5,224 |
United States | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 6 | 4 |
United States | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 390 | 521 |
United States | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 138 | 104 |
United States | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 2,743 | 2,521 |
United States | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 757 | 475 |
United States | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 900 | 660 |
United States | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 240 | 239 |
United States | Other, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 15 | 18 |
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 3,277 | 3,148 |
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 6 | 2 |
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 390 | 521 |
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 138 | 104 |
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 2,743 | 2,521 |
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Other, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 1,897 | 1,376 |
United States | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 2 |
United States | Significant Other Observable Inputs (Level 2) | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Other Observable Inputs (Level 2) | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Other Observable Inputs (Level 2) | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Other Observable Inputs (Level 2) | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 757 | 475 |
United States | Significant Other Observable Inputs (Level 2) | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 900 | 660 |
United States | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 240 | 239 |
United States | Significant Other Observable Inputs (Level 2) | Other, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 15 | 18 | 23
United States | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Unobservable Inputs (Level 3) | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Unobservable Inputs (Level 3) | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Unobservable Inputs (Level 3) | Real estate, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 0 | 0 | 0
United States | Significant Unobservable Inputs (Level 3) | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Unobservable Inputs (Level 3) | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Unobservable Inputs (Level 3) | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Unobservable Inputs (Level 3) | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
United States | Significant Unobservable Inputs (Level 3) | Insurance contracts, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 0 | 0 | 0
United States | Significant Unobservable Inputs (Level 3) | Other, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 15 | 18 |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 15 | 18 | 23
International |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 9,339 | 7,794 | 7,204
Fair Value of Plan Assets Excluding Net Asset Value Investments | 8,537 | 7,006 |
Investments measured at NAV practical expedient | 802 | 788 |
International | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 73 | 53 |
International | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 3,888 | 3,033 |
International | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 238 | 172 |
International | Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 2,344 | 2,027 |
International | Corporate obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 334 | 284 |
International | Fixed income obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 11 | 9 |
International | Real estate, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 3 | 7 |
International | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 660 | 565 |
International | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 268 | 237 |
International | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 119 | 92 |
International | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 55 | 50 |
International | Insurance contracts, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 537 | 471 |
International | Other, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 7 | 6 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 1,602 | 952 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 54 | 42 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 562 | 187 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 62 | 24 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 249 | 123 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Corporate obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 5 | 2 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Fixed income obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 7 | 6 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Real estate, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 660 | 565 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 2 | 2 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 1 | 0 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Insurance contracts, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Other, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 1 |
International | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 6,462 | 5,637 |
International | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 19 | 11 |
International | Significant Other Observable Inputs (Level 2) | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 3,326 | 2,846 |
International | Significant Other Observable Inputs (Level 2) | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 176 | 148 |
International | Significant Other Observable Inputs (Level 2) | Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 2,095 | 1,904 |
International | Significant Other Observable Inputs (Level 2) | Corporate obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 329 | 282 |
International | Significant Other Observable Inputs (Level 2) | Fixed income obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 4 | 3 |
International | Significant Other Observable Inputs (Level 2) | Real estate, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 1 | 3 |
International | Significant Other Observable Inputs (Level 2) | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Other Observable Inputs (Level 2) | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 266 | 235 |
International | Significant Other Observable Inputs (Level 2) | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 118 | 92 |
International | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 55 | 50 |
International | Significant Other Observable Inputs (Level 2) | Insurance contracts, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 67 | 59 |
International | Significant Other Observable Inputs (Level 2) | Other, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 6 | 4 |
International | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 473 | 417 | 400
International | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Unobservable Inputs (Level 3) | Developed markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Unobservable Inputs (Level 3) | Emerging markets equities, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Unobservable Inputs (Level 3) | Government and agency obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Unobservable Inputs (Level 3) | Corporate obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Unobservable Inputs (Level 3) | Fixed income obligations, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Unobservable Inputs (Level 3) | Real estate, Investment funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 2 | 4 |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 2 | 4 | 5
International | Significant Unobservable Inputs (Level 3) | Developed markets, Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Unobservable Inputs (Level 3) | Government and agency obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Unobservable Inputs (Level 3) | Corporate obligations, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Unobservable Inputs (Level 3) | Mortgage and asset backed-securities, Fixed income securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
International | Significant Unobservable Inputs (Level 3) | Insurance contracts, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 470 | 412 |
Fair Value of Plan Assets Excluding Net Asset Value Investments | 470 | 412 | 393
International | Significant Unobservable Inputs (Level 3) | Other, Other investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 1 | 1 |
Fair Value of Plan Assets Excluding Net Asset Value Investments | $ 1 | $ 1 | $ 2

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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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