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Fair Value of Financial Instruments - Assets and Liabilities Measured and Recorded at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in securities (classified within other current and non-current assets)	$ 20	$ 39
Other trust fund investments:
Other trust fund investments	1	1
Measured using net asset value practical expedient:
Total assets	1,993	1,792
Derivative liabilities:
Total liabilities	1,103	977
Commodity contracts
Derivative assets:
Derivative assets	1,170	1,042
Derivative liabilities:
Commodity contracts	1,103	977
Interest rate contracts
Derivative assets:
Derivative assets		39
Cash and cash equivalents
Nuclear trust fund investments:
Nuclear trust fund investments	17	19
U.S. government and federal agency obligations
Nuclear trust fund investments:
Nuclear trust fund investments	68	46
Federal agency mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	100	100
Commercial mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	29	22
Corporate debt securities
Nuclear trust fund investments:
Nuclear trust fund investments	109	96
Equity securities
Nuclear trust fund investments:
Nuclear trust fund investments	388	312
Measured using net asset value practical expedient:
Equity contracts	8	8
Foreign government fixed income securities
Nuclear trust fund investments:
Nuclear trust fund investments	5	4
Equity securities-nuclear trust fund investments
Measured using net asset value practical expedient:
Equity contracts	78	64
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)	0	2
Other trust fund investments:
Other trust fund investments	1	1
Measured using net asset value practical expedient:
Total assets	558	517
Derivative liabilities:
Total liabilities	143	224
Level 1 | Commodity contracts
Derivative assets:
Derivative assets	84	137
Derivative liabilities:
Commodity contracts	143	224
Level 1 | Interest rate contracts
Derivative assets:
Derivative assets		0
Level 1 | Cash and cash equivalents
Nuclear trust fund investments:
Nuclear trust fund investments	17	19
Level 1 | U.S. government and federal agency obligations
Nuclear trust fund investments:
Nuclear trust fund investments	68	46
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	388	312
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	0	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)	20	18
Other trust fund investments:
Other trust fund investments	0	0
Measured using net asset value practical expedient:
Total assets	1,156	1,075
Derivative liabilities:
Total liabilities	805	664
Level 2 | Commodity contracts
Derivative assets:
Derivative assets	893	796
Derivative liabilities:
Commodity contracts	805	664
Level 2 | Interest rate contracts
Derivative assets:
Derivative assets		39
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	0	0
Level 2 | Federal agency mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	100	100
Level 2 | Commercial mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	29	22
Level 2 | Corporate debt securities
Nuclear trust fund investments:
Nuclear trust fund investments	109	96
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	5	4
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	19
Other trust fund investments:
Other trust fund investments	0	0
Measured using net asset value practical expedient:
Total assets	193	128
Derivative liabilities:
Total liabilities	155	89
Level 3 | Commodity contracts
Derivative assets:
Derivative assets	193	109
Derivative liabilities:
Commodity contracts	155	89
Level 3 | Interest rate contracts
Derivative assets:
Derivative assets		0
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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