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Nuclear Decommissioning Trust Fund (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 692 | $ 610 |
Unrealized Gains | 276 | 217 |
Unrealized Losses | 2 | 4 |
Proceeds from sales of available-for-sale securities and the related realized gains and losses |  |  |
Realized gains | 22 | 26 | $ 21
Realized losses | 8 | 11 | 14
Proceeds from sale of securities | 501 | 510 | $ 631
Cash and cash equivalents |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | 47 | 25 |
Unrealized Gains | 0 | 0 |
Unrealized Losses | $ 0 | $ 0 |
Weighted- average maturities (in years) | 0 years | 0 years |
U.S. government and federal agency obligations |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 43 | $ 73 |
Unrealized Gains | 1 | 1 |
Unrealized Losses | $ 0 | $ 0 |
Weighted- average maturities (in years) | 11 years | 11 years |
Federal agency mortgage-backed securities |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 82 | $ 62 |
Unrealized Gains | 1 | 1 |
Unrealized Losses | $ 1 | $ 1 |
Weighted- average maturities (in years) | 23 years | 25 years |
Commercial mortgage-backed securities |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 13 | $ 17 |
Unrealized Gains | 0 | 0 |
Unrealized Losses | $ 0 | $ 1 |
Weighted- average maturities (in years) | 20 years | 26 years |
Corporate debt securities |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 99 | $ 84 |
Unrealized Gains | 2 | 1 |
Unrealized Losses | $ 1 | $ 2 |
Weighted- average maturities (in years) | 11 years | 11 years |
Equity securities |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 403 | $ 346 |
Unrealized Gains | 272 | 214 |
Unrealized Losses | $ 0 | $ 0 |
Weighted- average maturities (in years) | 0 years | 0 years |
Foreign government fixed income securities |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 5 | $ 3 |
Unrealized Gains | 0 | 0 |
Unrealized Losses | $ 0 | $ 0 |
Weighted- average maturities (in years) | 9 years | 9 years |
South Texas Project |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Ownership Interest (as a percent) | 44.00% |  |

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- Definition The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning facilities, whether such amount is presented as a separate caption or as a parenthetical disclosure on the balance sheet. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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