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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, 2018 | Dec. 31, 2017
Nuclear decommissioning trust fund |  |
Fair Value | $ 680 | $ 692
Unrealized Gains | 267 | 276
Unrealized Losses | 6 | 2
Cash and cash equivalents |  |
Nuclear decommissioning trust fund |  |
Fair Value | 16 | 47
Unrealized Gains | 0 | 0
Unrealized Losses | 0 | 0
U.S. government and federal agency obligations |  |
Nuclear decommissioning trust fund |  |
Fair Value | 56 | 43
Unrealized Gains | 1 | 1
Unrealized Losses | $ 1 | $ 0
Weighted-average Maturities (In years) | 11 years | 11 years
Federal agency mortgage-backed securities |  |
Nuclear decommissioning trust fund |  |
Fair Value | $ 92 | $ 82
Unrealized Gains | 0 | 1
Unrealized Losses | $ 2 | $ 1
Weighted-average Maturities (In years) | 23 years | 23 years
Commercial mortgage-backed securities |  |
Nuclear decommissioning trust fund |  |
Fair Value | $ 16 | $ 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 |  |
Fair Value | $ 100 | $ 99
Unrealized Gains | 1 | 2
Unrealized Losses | $ 2 | $ 1
Weighted-average Maturities (In years) | 11 years | 11 years
Equity securities |  |
Nuclear decommissioning trust fund |  |
Fair Value | $ 395 | $ 402
Unrealized Gains | 265 | 272
Unrealized Losses | 0 | 0
Foreign government fixed income securities |  |
Nuclear decommissioning trust fund |  |
Fair Value | 5 | 5
Unrealized Gains | 0 | 0
Unrealized Losses | $ 0 | $ 0
Weighted-average Maturities (In years) | 8 years | 9 years

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- Definition Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&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://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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