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Recently Adopted and Recently Issued Accounting Guidance (Details) - USD ($) $ in Millions | Jan. 01, 2019 | Mar. 31, 2019 | Dec. 31, 2018
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Summary Of Significant Accounting Policies [Line Items] |  |  |
Right-of-use assets classified in Other noncurrent assets |  | $ 302 |
Operating lease liability |  | $ 309 |
Adoption of accounting standards (B) | $ 73 |  |
Accounting Standards Update 2016-02 |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Right-of-use assets classified in Other noncurrent assets | 320 |  |
Operating lease liability | 320 |  |
Accounting Standards Update 2017-12 |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Adoption of accounting standards (B) | 2 |  |
Other Noncurrent Liabilities and Deferred Credits | Accounting Standards Update 2016-02 |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Effect on retained earnings | 119 |  |
Property, Plant and Equipment | Accounting Standards Update 2016-02 |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Effect on retained earnings | (24) |  |
Other Noncurrent Assets | Accounting Standards Update 2016-02 |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Effect on retained earnings | $ (22) |  |

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