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Nuclear Decommissioning Trust Fund - Summary of aggregate fair values and realized gains and losses (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Dec. 31, 2020
Nuclear decommissioning trust fund |  |
Fair Value | $ 909 | $ 890
Unrealized Gains | 418 | 398
Unrealized Losses | 3 | 0
Cash and cash equivalents |  |
Nuclear decommissioning trust fund |  |
Fair Value | 20 | 23
Unrealized Gains | 0 | 0
Unrealized Losses | 0 | 0
U.S. government and federal agency obligations |  |
Nuclear decommissioning trust fund |  |
Fair Value | 73 | 70
Unrealized Gains | 4 | 6
Unrealized Losses | $ 2 | $ 0
Weighted-average Maturities (In years) | 12 years | 10 years
Federal agency mortgage-backed securities |  |
Nuclear decommissioning trust fund |  |
Fair Value | $ 78 | $ 89
Unrealized Gains | 3 | 4
Unrealized Losses | $ 0 | $ 0
Weighted-average Maturities (In years) | 23 years | 24 years
Commercial mortgage-backed securities |  |
Nuclear decommissioning trust fund |  |
Fair Value | $ 40 | $ 36
Unrealized Gains | 1 | 2
Unrealized Losses | $ 0 | $ 0
Weighted-average Maturities (In years) | 28 years | 27 years
Corporate debt securities |  |
Nuclear decommissioning trust fund |  |
Fair Value | $ 136 | $ 144
Unrealized Gains | 7 | 13
Unrealized Losses | $ 1 | $ 0
Weighted-average Maturities (In years) | 12 years | 12 years
Equity securities |  |
Nuclear decommissioning trust fund |  |
Fair Value | $ 555 | $ 521
Unrealized Gains | 403 | 372
Unrealized Losses | 0 | 0
Foreign government fixed income securities |  |
Nuclear decommissioning trust fund |  |
Fair Value | 7 | 7
Unrealized Gains | 0 | 1
Unrealized Losses | $ 0 | $ 0
Weighted-average Maturities (In years) | 9 years | 10 years

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + 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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