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Nuclear Decommissioning Trust Fund (Tables) | 12 Months Ended
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Dec. 31, 2017
Nuclear Decommissioning Trust Fund Disclosure [Abstract] |
Summary of Aggregate Fair Values and Unrealized Gains And Losses (Including Other-Than-Temporary Impairments) for the Securities Held in The Nuclear Decommissioning Trust Fund | The following table summarizes the aggregate fair values and unrealized gains and losses (including other-than-temporary impairments) for the securities held in the trust funds, as well as information about the contractual maturities of those securities.
| As of December 31, 2017 |  | As of December 31, 2016
(In millions, except otherwise noted) | Fair Value |  | Unrealized Gains |  | Unrealized Losses |  | Weighted- average maturities (in years) |  | Fair Value |  | Unrealized Gains |  | Unrealized Losses |  | Weighted- average maturities (in years)
Cash and cash equivalents | $ | 47 |  |  | $ | — |  |  | $ | — |  |  | — |  |  | $ | 25 |  |  | $ | — |  |  | $ | — |  |  | — |
U.S. government and federal agency obligations | 43 |  |  | 1 |  |  | — |  |  | 11 |  |  | 73 |  |  | 1 |  |  | — |  |  | 11 |
Federal agency mortgage-backed securities | 82 |  |  | 1 |  |  | 1 |  |  | 23 |  |  | 62 |  |  | 1 |  |  | 1 |  |  | 25 |
Commercial mortgage-backed securities | 13 |  |  | — |  |  | — |  |  | 20 |  |  | 17 |  |  | — |  |  | 1 |  |  | 26 |
Corporate debt securities | 99 |  |  | 2 |  |  | 1 |  |  | 11 |  |  | 84 |  |  | 1 |  |  | 2 |  |  | 11 |
Equity securities | 403 |  |  | 272 |  |  | — |  |  | — |  |  | 346 |  |  | 214 |  |  | — |  |  | — |
Foreign government fixed income securities | 5 |  |  | — |  |  | — |  |  | 9 |  |  | 3 |  |  | — |  |  | — |  |  | 9 |
Total | $ | 692 |  |  | $ | 276 |  |  | $ | 2 |  |  |  |  |  | $ | 610 |  |  | $ | 217 |  |  | $ | 4 |  |  |  |
Summary of Proceeds From Sales of Available-For-Sale securities and the related realized gains and losses | The following table summarizes proceeds from sales of available-for-sale securities and the related realized gains and losses from these sales. The cost of securities sold is determined using the specific identification method.
| Year Ended December 31,
| 2017 |  | 2016 |  | 2015
| (In millions)
Realized gains | $ | 22 |  |  | $ | 26 |  |  | $ | 21 |
Realized losses | 8 |  |  | 11 |  |  | 14 |
Proceeds from sale of securities | 501 |  |  | 510 |  |  | 631 |

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- Definition Disclosure related to investments held in a trust fund to pay for the costs of decommissioning facilities. + References No definition available. + Details
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- Definition Tabular disclosure of the proceeds from sales of available-for-sale securities and the gross realized gains and gross realized losses that have been included in earnings as a result of those sales. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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