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Restructuring and Separation Costs - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended | 21 Months Ended |
--- | --- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Sep. 30, 2018 | Dec. 31, 2018
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |
Restructuring and separation costs | $ 5 | $ 118 | $ 38 | $ 161 |  |  |
IT Restructuring |  |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |
Restructuring and separation costs | 3 |  |  |  |  |  |
IT Restructuring | Scenario, Forecast |  |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |
Expected restructuring costs |  |  |  |  |  |  | $ 6
2017 Restructuring Plan |  |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |
Restructuring and separation costs | $ 2 | $ 118 | $ 35 | $ 161 | $ 234 | $ 269 |

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- Definition Amount, after cash payment, of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + Details
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