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Revenues (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total operating revenues | $ 2,880 | $ 3,048 | $ 2,658 | $ 2,951 | $ 2,796 | $ 3,017 | $ 2,645 | $ 2,946 | $ 11,537 | $ 11,404 | $ 11,107
Total revenue from contracts with customers |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 11,020 |  |
Retail |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 9,508 |  |
Retail | Residential |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 3,945 |  |
Retail | Commercial and industrial (C&I) |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 5,423 |  |
Retail | Other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 140 |  |
Wholesale |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 791 |  |
Transmission |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 523 |  |
Other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 198 |  |
Alternative revenue and other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  |  |  | 517 |  |
Regulated Electric | Total revenue from contracts with customers |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 9,339 |  |
Regulated Electric | Retail |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 7,927 |  |
Regulated Electric | Retail | Residential |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 2,919 |  |
Regulated Electric | Retail | Commercial and industrial (C&I) |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 4,874 |  |
Regulated Electric | Retail | Other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 134 |  |
Regulated Electric | Wholesale |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 791 |  |
Regulated Electric | Transmission |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 523 |  |
Regulated Electric | Other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 98 |  |
Regulated Electric | Alternative revenue and other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  |  |  | 380 |  |
Regulated Natural Gas | Total revenue from contracts with customers |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 1,612 |  |
Regulated Natural Gas | Retail |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 1,512 |  |
Regulated Natural Gas | Retail | Residential |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 988 |  |
Regulated Natural Gas | Retail | Commercial and industrial (C&I) |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 524 |  |
Regulated Natural Gas | Retail | Other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 0 |  |
Regulated Natural Gas | Wholesale |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 0 |  |
Regulated Natural Gas | Transmission |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 0 |  |
Regulated Natural Gas | Other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 100 |  |
Regulated Natural Gas | Alternative revenue and other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  |  |  | 127 |  |
All Other | Total revenue from contracts with customers |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 69 |  |
All Other | Retail |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 69 |  |
All Other | Retail | Residential |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 38 |  |
All Other | Retail | Commercial and industrial (C&I) |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 25 |  |
All Other | Retail | Other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 6 |  |
All Other | Wholesale |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 0 |  |
All Other | Transmission |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 0 |  |
All Other | Other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue from contracts with customers |  |  |  |  |  |  |  |  | 0 |  |
All Other | Alternative revenue and other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  |  |  | 10 |  |
Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  |  |  | 11,537 |  |
Operating Segments | Regulated Electric |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  |  |  | 9,719 |  |
Operating Segments | Regulated Natural Gas |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  |  |  | 1,739 |  |
Operating Segments | All Other |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total operating revenues |  |  |  |  |  |  |  |  | $ 79 |  |

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