Fair Value of Financial Instruments - Assets and Liabilities Measured and Recorded at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 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)	$ 38	$ 39
U.S. government and federal agency obligations	1	1
Total assets	2,072	1,792
Derivative liabilities	1,152	977
Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1,276	1,042
Derivative liabilities	1,152	977
Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		39
Cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	25	19
U.S. government and federal agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	57	46
Federal agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	92	100
Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	29	22
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	102	96
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	366	312
Equity contracts	9	8
Foreign government fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	4	4
Equity securities — nuclear trust fund investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity contracts	73	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
U.S. government and federal agency obligations	1	1
Total assets	580	517
Derivative liabilities	245	224
Level 1 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	131	137
Derivative liabilities	245	224
Level 1 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0
Level 1 | Cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	25	19
Level 1 | U.S. government and federal agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	57	46
Level 1 | Federal agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 1 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 1 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	366	312
Level 1 | Foreign government fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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)	19	18
U.S. government and federal agency obligations	0	0
Total assets	1,016	1,075
Derivative liabilities	629	664
Level 2 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	770	796
Derivative liabilities	629	664
Level 2 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		39
Level 2 | Cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 2 | U.S. government and federal agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 2 | Federal agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	92	100
Level 2 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	29	22
Level 2 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	102	96
Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 2 | Foreign government fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	4	4
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)	19	19
U.S. government and federal agency obligations	0	0
Total assets	394	128
Derivative liabilities	278	89
Level 3 | Commodity contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	375	109
Derivative liabilities	278	89
Level 3 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0
Level 3 | Cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 3 | U.S. government and federal agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 3 | Federal agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 3 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 3 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	0	0
Level 3 | Foreign government fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Nuclear trust fund investments	$ 0	$ 0