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Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Property, Plant and Equipment [Abstract] |  |  |
Depreciation expense expressed as a percentage of average depreciable property | 3.10% | 3.10% | 2.90%
Nuclear Decommissioning [Abstract] |  |  |
Minimum amount of time between nuclear decommissioning studies (in years) | 3 years |  |
Cash and Cash Equivalents [Abstract] |  |  |
Maximum number of months of remaining maturity at time of purchase to consider investments in certain instruments as cash equivalents | 3 months |  |
Accounts, Notes, Loans and Financing Receivable [Abstract] |  |  |
Allowance for bad debts | $ 55 | $ 52 |
Alternative Revenue Programs [Abstract] |  |  |
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs | 24 months |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventories | $ 548 | 610 |
Supplies [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventories | 271 | 311 |
Public Utilities, Inventory, Fuel [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventories | 170 | 186 |
Public Utilities, Inventory, Natural Gas [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventories | $ 107 | $ 113 |

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