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Investment in Cenovus Energy (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 5 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2018 | May 17, 2017 | Sep. 30, 2018 | Jan. 01, 2018 | Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items] |  |  |  |  |
Investment in Cenovus Energy | $ 2,086 |  | $ 2,086 |  | $ 1,899
Accumulated Other Comprehensive Income , Net of Tax | (5,442) |  | (5,442) |  | (5,518)
Adjustments for New Accounting Pronouncement [Member] |  |  |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |  |  |
Accumulated Other Comprehensive Income , Net of Tax |  |  |  | $ (5,460) | (5,518)
Common Stock [Member] |  |  |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |  |  |
Ivnestment in Equity Security Cost |  | $ 1,960 |  |  |
Common Stock [Member] | Foster Creek Christina Lake (FCCL) and western Canada gas properties [Member] |  |  |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |  |  |
Shares Exchange for Assets Held for Sale |  | 208 |  |  |
Investment Owned, Common Stock, percent of Issued and Outstanding |  | 16.90% |  |  |
Investment In Equity Security Fair Value | 2,090 | $ 1,960 | $ 2,090 |  | $ 1,900
Investment Owned, Common Stock, price per share |  | $ 9.41 | $ 10.03 |  |
Unrealized Gain (Loss) on Securities | $ (73) |  | $ 187 |  |
Common Stock [Member] | ASU 2016-01 Accounting for Cenovus Shares [Member] | Foster Creek Christina Lake (FCCL) and western Canada gas properties [Member] |  |  |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |  |  |
Accumulated Other Comprehensive Income , Net of Tax |  |  |  | $ (58) |

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