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Nuclear Decommissioning Trust Fund - Summary of Aggregate Fair Values and Realized Gains and Losses (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Nuclear decommissioning trust fund disclosure |  |
Fair Value | $ 794 | $ 663
Unrealized Gains | 338 | 234
Unrealized Losses | 1 | 6
Cash and cash equivalents |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | 17 | 19
Unrealized Gains | 0 | 0
Unrealized Losses | 0 | 0
U.S. government and federal agency obligations |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | 68 | 46
Unrealized Gains | 4 | 1
Unrealized Losses | $ 0 | $ 0
Weighted- average maturities (in years) | 11 years | 12 years
Federal agency mortgage-backed securities |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | $ 100 | $ 100
Unrealized Gains | 3 | 1
Unrealized Losses | $ 0 | $ 2
Weighted- average maturities (in years) | 24 years | 23 years
Commercial mortgage-backed securities |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | $ 29 | $ 22
Unrealized Gains | 1 | 0
Unrealized Losses | $ 1 | $ 1
Weighted- average maturities (in years) | 24 years | 22 years
Corporate debt securities |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | $ 109 | $ 96
Unrealized Gains | 6 | 1
Unrealized Losses | $ 0 | $ 2
Weighted- average maturities (in years) | 11 years | 11 years
Equity securities |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | $ 466 | $ 376
Unrealized Gains | 324 | 231
Unrealized Losses | 0 | 1
Foreign government fixed income securities |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | 5 | 4
Unrealized Gains | 0 | 0
Unrealized Losses | $ 0 | $ 0
Weighted- average maturities (in years) | 10 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://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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- Definition The proportionate share of interest (percent) in a jointly owned utility plant. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 10.C) -URI http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033 + Details
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