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Nuclear Decommissioning Trust Fund (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 663 | $ 692 |
Unrealized Gains | 234 | 276 |
Unrealized Losses | 6 | 2 |
Proceeds from sales of available-for-sale securities and the related realized gains and losses |  |  |
Realized gains | 17 | 22 | $ 26
Realized (losses) | (13) | (8) | (11)
Proceeds from sale of securities | 513 | 501 | $ 510
Cash and cash equivalents |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | 19 | 47 |
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 | $ 46 | $ 43 |
Unrealized Gains | 1 | 1 |
Unrealized Losses | $ 0 | $ 0 |
Weighted- average maturities (in years) | 12 years | 11 years |
Federal agency mortgage-backed securities |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 100 | $ 82 |
Unrealized Gains | 1 | 1 |
Unrealized Losses | $ 2 | $ 1 |
Weighted- average maturities (in years) | 23 years | 23 years |
Commercial mortgage-backed securities |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 22 | $ 14 |
Unrealized Gains | 0 | 0 |
Unrealized Losses | $ 1 | $ 0 |
Weighted- average maturities (in years) | 22 years | 20 years |
Corporate debt securities |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 96 | $ 99 |
Unrealized Gains | 1 | 2 |
Unrealized Losses | $ 2 | $ 1 |
Weighted- average maturities (in years) | 11 years | 11 years |
Equity securities |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 376 | $ 402 |
Unrealized Gains | 231 | 272 |
Unrealized Losses | $ 1 | $ 0 |
Weighted- average maturities (in years) | 0 years | 0 years |
Foreign government fixed income securities |  |  |
Nuclear decommissioning trust fund disclosure |  |  |
Fair Value | $ 4 | $ 5 |
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 Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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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- 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://fasb.org/us-gaap/role/ref/legacyRef -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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