| **Basis of Presentation and Significant Accounting Policies - Impact of Lease Standard Adoption (Details) - USD ($) $ in Millions** | Mar. 31, 2019 | [1]     | Jan. 01, 2019 | Dec. 31, 2018 | [1] |
|-------------------------------------------------------------------------------------------------------------------------------------|---------------|---------|---------------|---------------|-----|
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                                    |          |    |          |
| Other current assets                                                                                                                | $ 2,567       | $ 2,460 | $ 2,461       |
| Noncurrent deferred tax assets and other noncurrent tax assets                                                                      | 1,946         | 1,913   | 1,924         |
| Other noncurrent assets                                                                                                             | 4,333         | 4,149   | 2,799         |
| Other current liabilities                                                                                                           | 10,276        | 11,011  | 10,753        |
| Other noncurrent liabilities                                                                                                        | 6,346         | 6,910   | 5,850         |
| Retained earnings                                                                                                                   | $ 93,388      | 89,574  | $ 89,554      |
| Accounting Standards Update 2016-02 [Member]                                                                                        |          |    |          |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                                    |          |    |          |
| Other current assets                                                                                                                |          | (1)     |          |
| Noncurrent deferred tax assets and other noncurrent tax assets                                                                      |          | (11)    |          |
| Other noncurrent assets                                                                                                             |          | 1,351   |          |
| Other current liabilities                                                                                                           |          | 258     |          |
| Other noncurrent liabilities                                                                                                        |          | 1,060   |          |
| Retained earnings                                                                                                                   |          | $ 20    |          |
|                                                                                                                                     |
| [1]Amounts may not add due to rounding.                                                                                             |

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