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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, 2020 | Dec. 31, 2019
Nuclear decommissioning trust fund disclosure |  |
Fair Value | $ 890 | $ 794
Unrealized Gains | 398 | 338
Unrealized Losses | 0 | 1
Cash and cash equivalents |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | 23 | 17
Unrealized Gains | 0 | 0
Unrealized Losses | 0 | 0
U.S. government and federal agency obligations |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | 70 | 68
Unrealized Gains | 6 | 4
Unrealized Losses | $ 0 | $ 0
Weighted- average maturities (in years) | 10 years | 11 years
Federal agency mortgage-backed securities |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | $ 89 | $ 100
Unrealized Gains | 4 | 3
Unrealized Losses | $ 0 | $ 0
Weighted- average maturities (in years) | 24 years | 24 years
Commercial mortgage-backed securities |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | $ 36 | $ 29
Unrealized Gains | 2 | 1
Unrealized Losses | $ 0 | $ 1
Weighted- average maturities (in years) | 27 years | 24 years
Corporate debt securities |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | $ 144 | $ 109
Unrealized Gains | 13 | 6
Unrealized Losses | $ 0 | $ 0
Weighted- average maturities (in years) | 12 years | 11 years
Equity securities |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | $ 521 | $ 466
Unrealized Gains | 372 | 324
Unrealized Losses | 0 | 0
Foreign government fixed income securities |  |
Nuclear decommissioning trust fund disclosure |  |
Fair Value | 7 | 5
Unrealized Gains | 1 | 0
Unrealized Losses | $ 0 | $ 0
Weighted- average maturities (in years) | 10 years | 10 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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