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Balance Sheet Components (Tables) | 12 Months Ended
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Dec. 31, 2017
Balance Sheet Components [Abstract] |
Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] | The following is the activity in the allowance for uncollectible accounts (in millions):
| Years Ended December 31,
| 2017 |  | 2016 |  | 2015
Balance as of beginning of year | $ | 6 |  |  | $ | 6 |  |  | $ | 6 |
Increase in provision | 6 |  |  | 5 |  |  | 6 |
Amounts written off, less recoveries | (6 | ) |  | (5 | ) |  | (6 | )
Balance as of end of year | $ | 6 |  |  | $ | 6 |  |  | $ | 6 |
Investments held in trust [Table Text Block] | The trusts are comprised of the following investments as of December 31 (in millions):
| Nuclear Decommissioning Trust |  | Non-Qualified Benefit Plan Trust
| 2017 |  | 2016 |  | 2017 |  | 2016
Cash equivalents | $ | 25 |  |  | $ | 21 |  |  | $ | 1 |  |  | $ | 1 |
Marketable securities, at fair value: |  |  |  |  |  |  |
Equity securities | — |  |  | — |  |  | 7 |  |  | 6 |
Debt securities | 17 |  |  | 20 |  |  | 1 |  |  | 1 |
Insurance contracts, at cash surrender value | — |  |  | — |  |  | 28 |  |  | 26 |
| $ | 42 |  |  | $ | 41 |  |  | $ | 37 |  |  | $ | 34 |
Schedule of Other Assets and Other Liabilities [Table Text Block] | Other current assets and Accrued expenses and other current liabilities consist of the following (in millions):
| As of December 31,
| 2017 |  | 2016
Other current assets: |  |  |
Prepaid expenses | $ | 50 |  |  | $ | 48 |
Margin deposits | 11 |  |  | 8 |
Assets from price risk management activities | 6 |  |  | 18 |
Other | 6 |  |  | 3 |
| $ | 73 |  |  | $ | 77 |
Accrued expenses and other current liabilities: |  |  |
Regulatory liabilities—current | $ | 31 |  |  | $ | 51 |
Accrued employee compensation and benefits | 60 |  |  | 52 |
Accrued dividends payable | 31 |  |  | 30 |
Accrued interest payable | 27 |  |  | 25 |
Accrued taxes payable | 31 |  |  | 25 |
Other | 61 |  |  | 71 |
| $ | 241 |  |  | $ | 254 |

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- Definition investments held in trust [Table Text Block] + References No definition available. + Details
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- Definition Tabular disclosure of assets and liabilities, classified as other. + References No definition available. + Details
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- Definition The entire disclosure for any allowance and reserve accounts (their beginning and ending balances, as well as a reconciliation by type of activity during the period). Alternatively, disclosure of the required information may be within the footnotes to the financial statements or a supplemental schedule to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -URI http://asc.fasb.org/topic&trid=2122369 + Details
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