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Acquisitions and Dispositions of Businesses - Dispositions (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) disposal | Dec. 31, 2017 USD ($) disposal | Dec. 31, 2016 USD ($) disposal
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Disposal of business |  | $ (6) | $ (16) | $ 39
Disposal Group, Not Discontinued Operations |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Number of dispositions | disposal |  | 4 | 9 | 5
Disposal Group, Held-for-sale or Disposed |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Disposal group assets | $ 47 |  |  |
Disposal group liabilities | 41 |  |  |
Amortization and Impairment of Intangible Assets | Disposal Group, Held-for-sale or Disposed |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Noncash impairment charge | $ 176 |  |  |

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