Nuclear Decommissioning Trust Fund (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nuclear decommissioning trust fund disclosure
Fair Value	$ 663	$ 692
Unrealized Gains	234	276
Unrealized Losses	6	2
Proceeds from sales of available-for-sale securities and the related realized gains and losses
Realized gains	17	22	$ 26
Realized (losses)	(13)	(8)	(11)
Proceeds from sale of securities	513	501	$ 510
Cash and cash equivalents
Nuclear decommissioning trust fund disclosure
Fair Value	19	47
Unrealized Gains	0	0
Unrealized Losses	$ 0	$ 0
Weighted- average maturities (in years)	0 years	0 years
U.S. government and federal agency obligations
Nuclear decommissioning trust fund disclosure
Fair Value	$ 46	$ 43
Unrealized Gains	1	1
Unrealized Losses	$ 0	$ 0
Weighted- average maturities (in years)	12 years	11 years
Federal agency mortgage-backed securities
Nuclear decommissioning trust fund disclosure
Fair Value	$ 100	$ 82
Unrealized Gains	1	1
Unrealized Losses	$ 2	$ 1
Weighted- average maturities (in years)	23 years	23 years
Commercial mortgage-backed securities
Nuclear decommissioning trust fund disclosure
Fair Value	$ 22	$ 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 disclosure
Fair Value	$ 96	$ 99
Unrealized Gains	1	2
Unrealized Losses	$ 2	$ 1
Weighted- average maturities (in years)	11 years	11 years
Equity securities
Nuclear decommissioning trust fund disclosure
Fair Value	$ 376	$ 402
Unrealized Gains	231	272
Unrealized Losses	$ 1	$ 0
Weighted- average maturities (in years)	0 years	0 years
Foreign government fixed income securities
Nuclear decommissioning trust fund disclosure
Fair Value	$ 4	$ 5
Unrealized Gains	0	0
Unrealized Losses	$ 0	$ 0
Weighted- average maturities (in years)	9 years	9 years
South Texas Project
Nuclear decommissioning trust fund disclosure
Ownership Interest (as a percent)	44.00%

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Decommissioning Fund Investments, Unrealized Loss

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

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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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.

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The proportionate share of interest (percent) in a jointly owned utility plant.

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