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Basis of Presentation (Details) retail_customers in Thousands, mi² in Thousands, $ in Millions | 3 Months Ended
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Mar. 31, 2019 USD ($) mi² retail_customers | Mar. 31, 2018 USD ($)
Basis of Presentation [Abstract] |  |
Service Area Sq Miles | mi² | 4 |
Incorporated Cities | 51 |
Number of Retail Customers | retail_customers | 887 |
Percent of state population | 46.00% |
Alternative revenue programs, net of amortization | $ 3 | $ (2)
Reclassification of stranded tax effects due to Tax Reform | 2 |
Revenues | 573 | $ 493
AOCI Attributable to Parent [Member] |  |
Reclassification of stranded tax effects due to Tax Reform | 2 |
Property, Plant and Equipment [Member] |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | (178) |
Accounting Standards Update 2016-02 [Member] |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 44 |
Accounting Standards Update 2016-02 [Member] | Property, Plant and Equipment [Member] |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 2 |
Building [Member] | Property, Plant and Equipment [Member] |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 131 |
Assets Held under Capital Leases [Member] | Property, Plant and Equipment [Member] |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | $ 49 |

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- Definition Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in estimate of transaction price which (increases) decreases obligation to transfer good or service to customer for which consideration from customer has been received or is due. Includes, but is not limited to, change in assessment of whether estimate of variable consideration is constrained. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130551-203045 + Details
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