Fair Value of Financial Instruments - Assets and Liabilities Measured and Recorded at Fair Value (Details) - USD ($) $ in Millions	Mar. 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)	$ 13	$ 20
Other trust fund investments:
U.S. government and federal agency obligations	1	1
Measured using net asset value practical expedient:
Total assets	2,040	1,993
Derivative liabilities:
Derivative liabilities	1,202	1,103
Commodity contracts
Derivative assets:
Derivative assets	1,318	1,170
Derivative liabilities:
Derivative liabilities	1,202	1,103
Cash and cash equivalents
Nuclear trust fund investments:
Nuclear trust fund investments	21	17
U.S. government and federal agency obligations
Nuclear trust fund investments:
Nuclear trust fund investments	57	68
Federal agency mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	93	100
Commercial mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	33	29
Corporate debt securities
Nuclear trust fund investments:
Nuclear trust fund investments	125	109
Equity securities
Nuclear trust fund investments:
Nuclear trust fund investments	308	388
Measured using net asset value practical expedient:
Equity contracts	6	8
Foreign government fixed income securities
Nuclear trust fund investments:
Nuclear trust fund investments	5	5
Equity securities — nuclear trust fund investments
Measured using net asset value practical expedient:
Equity contracts	60	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)	0	0
Other trust fund investments:
U.S. government and federal agency obligations	1	1
Measured using net asset value practical expedient:
Total assets	497	558
Derivative liabilities:
Derivative liabilities	193	143
Level 1 | Commodity contracts
Derivative assets:
Derivative assets	111	84
Derivative liabilities:
Derivative liabilities	193	143
Level 1 | Cash and cash equivalents
Nuclear trust fund investments:
Nuclear trust fund investments	21	17
Level 1 | U.S. government and federal agency obligations
Nuclear trust fund investments:
Nuclear trust fund investments	56	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	308	388
Level 1 | Foreign government fixed income securities
Nuclear trust fund investments:
Nuclear trust fund investments	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)	13	20
Other trust fund investments:
U.S. government and federal agency obligations	0	0
Measured using net asset value practical expedient:
Total assets	1,285	1,156
Derivative liabilities:
Derivative liabilities	890	805
Level 2 | Commodity contracts
Derivative assets:
Derivative assets	1,015	893
Derivative liabilities:
Derivative liabilities	890	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	93	100
Level 2 | Commercial mortgage-backed securities
Nuclear trust fund investments:
Nuclear trust fund investments	33	29
Level 2 | Corporate debt securities
Nuclear trust fund investments:
Nuclear trust fund investments	125	109
Level 2 | Equity securities
Nuclear trust fund investments:
Nuclear trust fund investments	0	0
Level 2 | Foreign government fixed income securities
Nuclear trust fund investments:
Nuclear trust fund investments	5	5
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:
U.S. government and federal agency obligations	0	0
Measured using net asset value practical expedient:
Total assets	192	193
Derivative liabilities:
Derivative liabilities	119	155
Level 3 | Commodity contracts
Derivative assets:
Derivative assets	192	193
Derivative liabilities:
Derivative liabilities	119	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
Level 3 | Foreign government fixed income securities
Nuclear trust fund investments:
Nuclear trust fund investments	$ 0	$ 0