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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

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

+ References

No definition available.

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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

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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

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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

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