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Fair Value of Financial Instruments - Assets and Liabilities Measured and Recorded at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in securities (classified within other current and non-current assets)	$ 25	$ 20
Other trust fund investments:
Other trust fund investments	1	1
Measured using net asset value practical expedient:
Total assets	1,745	1,993
Derivative liabilities:
Total liabilities	884	1,103
Commodity contracts
Derivative assets:
Derivative assets	821	1,170
Derivative liabilities:
Commodity contracts	884	1,103
Cash and cash equivalents
Nuclear trust fund investments:
Nuclear trust fund investments	23	17
U.S. government and federal agency obligations
Nuclear trust fund investments:
Nuclear trust fund investments	70	68
Federal agency mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	89	100
Commercial mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	36	29
Corporate debt securities
Nuclear trust fund investments:
Nuclear trust fund investments	144	109
Equity securities
Nuclear trust fund investments:
Nuclear trust fund investments	434	388
Measured using net asset value practical expedient:
Equity contracts	8	8
Foreign government fixed income securities
Nuclear trust fund investments:
Nuclear trust fund investments	7	5
Equity securities-nuclear trust fund investments
Measured using net asset value practical expedient:
Equity contracts	87	78
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in securities (classified within other current and non-current assets)	10	0
Other trust fund investments:
Other trust fund investments	1	1
Measured using net asset value practical expedient:
Total assets	597	558
Derivative liabilities:
Total liabilities	86	143
Level 1 | Commodity contracts
Derivative assets:
Derivative assets	59	84
Derivative liabilities:
Commodity contracts	86	143
Level 1 | Cash and cash equivalents
Nuclear trust fund investments:
Nuclear trust fund investments	23	17
Level 1 | U.S. government and federal agency obligations
Nuclear trust fund investments:
Nuclear trust fund investments	69	68
Level 1 | Federal agency mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 1 | Commercial mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 1 | Corporate debt securities
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 1 | Equity securities
Nuclear trust fund investments:
Nuclear trust fund investments	434	388
Measured using net asset value practical expedient:
Equity contracts	0	0
Level 1 | Foreign government fixed income securities
Nuclear trust fund investments:
Nuclear trust fund investments	1	0
Level 1 | Equity securities-nuclear trust fund investments
Measured using net asset value practical expedient:
Equity contracts	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in securities (classified within other current and non-current assets)	15	20
Other trust fund investments:
Other trust fund investments	0	0
Measured using net asset value practical expedient:
Total assets	914	1,156
Derivative liabilities:
Total liabilities	643	805
Level 2 | Commodity contracts
Derivative assets:
Derivative assets	623	893
Derivative liabilities:
Commodity contracts	643	805
Level 2 | Cash and cash equivalents
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 2 | U.S. government and federal agency obligations
Nuclear trust fund investments:
Nuclear trust fund investments	1	0
Level 2 | Federal agency mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	89	100
Level 2 | Commercial mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	36	29
Level 2 | Corporate debt securities
Nuclear trust fund investments:
Nuclear trust fund investments	144	109
Level 2 | Equity securities
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Measured using net asset value practical expedient:
Equity contracts	0	0
Level 2 | Foreign government fixed income securities
Nuclear trust fund investments:
Nuclear trust fund investments	6	5
Level 2 | Equity securities-nuclear trust fund investments
Measured using net asset value practical expedient:
Equity contracts	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in securities (classified within other current and non-current assets)	0	0
Other trust fund investments:
Other trust fund investments	0	0
Measured using net asset value practical expedient:
Total assets	139	193
Derivative liabilities:
Total liabilities	155	155
Level 3 | Commodity contracts
Derivative assets:
Derivative assets	139	193
Derivative liabilities:
Commodity contracts	155	155
Level 3 | Cash and cash equivalents
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 3 | U.S. government and federal agency obligations
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 3 | Federal agency mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 3 | Commercial mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 3 | Corporate debt securities
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 3 | Equity securities
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Measured using net asset value practical expedient:
Equity contracts	0	0
Level 3 | Foreign government fixed income securities
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 3 | Equity securities-nuclear trust fund investments
Measured using net asset value practical expedient:
Equity contracts	$ 0	$ 0

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Decommission fund to pay for the costs of decontaminating and decommissioning of facilities through collection of revenues derived from utility assessments and government appropriations. Decommission fund investment for the process whereby a power station, at the end of its economic life, is taken permanently out of service and its site made available for other purposes. In the case of a nuclear station this comprises three different states of clearance. Immediately after the final closure, radioactive material such as nuclear fuel and operational waste is removed and the buildings surrounding the reactor shield are dismantled and finally the reactor itself is dismantled.

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