| **Income Taxes (Tables)**            | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                      |
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| **Income Tax Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                |
| Income Tax Provision (Benefit)       | The income tax expense (benefit) consists of the following: Three Months Ended September 30, Nine Months Ended September 30,(millions, except percentages)2018 2017 2018 2017Current$45 $22 $194 $71Deferred14 (115) (150) (988)Total Income Tax Expense (Benefit)$59 $(93) $44 $(917)Effective Tax Rate19.2% 44.7% 5.1% 36.9% |

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| - References
No definition available.
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| - Definition
Tabular disclosure of the components of income tax expense attributable to continuing operations for each year presented including, but not limited to: current tax expense (benefit), deferred tax expense (benefit), investment tax credits, government grants, the benefits of operating loss carryforwards, tax expense that results from allocating certain tax benefits either directly to contributed capital or to reduce goodwill or other noncurrent intangible assets of an acquired entity, adjustments of a deferred tax liability or asset for enacted changes in tax laws or rates or a change in the tax status of the entity, and adjustments of the beginning-of-the-year balances of a valuation allowance because of a change in circumstances that causes a change in judgment about the realizability of the related deferred tax asset in future years.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319
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