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Nuclear Decommissioning Trust Fund - Summary of proceeds from sales of available-for-sale securities and related gains and losses (Details) - USD ($) $ in Millions | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017
Nuclear Decommissioning Trust Fund Disclosure [Abstract] |  |
Realized gains | $ 7 | $ 3
Realized losses | 6 | 3
Proceeds from sale of securities | $ 303 | $ 277

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- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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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