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Nuclear Decommissioning Trust Fund - Summary of aggregate fair values and realized gains and losses (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Nuclear decommissioning trust fund
Fair Value	$ 719	$ 692
Unrealized Gains	298	276
Unrealized Losses	7	2
Cash and cash equivalents
Nuclear decommissioning trust fund
Fair Value	21	47
Unrealized Gains	0	0
Unrealized Losses	0	0
U.S. government and federal agency obligations
Nuclear decommissioning trust fund
Fair Value	41	43
Unrealized Gains	1	1
Unrealized Losses	$ 1	$ 0
Weighted-average Maturities (In years)	12 years	11 years
Federal agency mortgage-backed securities
Nuclear decommissioning trust fund
Fair Value	$ 88	$ 82
Unrealized Gains	0	1
Unrealized Losses	$ 3	$ 1
Weighted-average Maturities (In years)	23 years	23 years
Commercial mortgage-backed securities
Nuclear decommissioning trust fund
Fair Value	$ 19	$ 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	$ 112	$ 99
Unrealized Gains	1	2
Unrealized Losses	$ 2	$ 1
Weighted-average Maturities (In years)	10 years	11 years
Equity securities
Nuclear decommissioning trust fund
Fair Value	$ 434	$ 402
Unrealized Gains	296	272
Unrealized Losses	0	0
Foreign government fixed income securities
Nuclear decommissioning trust fund
Fair Value	4	5
Unrealized Gains	0	0
Unrealized Losses	$ 0	$ 0
Weighted-average Maturities (In years)	9 years	9 years

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The weighted-average maturity period for securities of nuclear decommissioning trust funds assets.

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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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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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

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-Topic 410
-SubTopic 20
-Section 50
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