Fair Value of Financial Instruments - Assets and Liabilities Measured and Recorded at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	$ 798	$ 1,248
Derivative liabilities	752	1,376
Commodity contracts
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	745	1,199
Derivative liabilities	693	1,288
Interest rate contracts
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	53	49
Derivative liabilities	59	88
Fair Value, Measurements, Recurring
Investment in available-for-sale securities (classified within other non-current assets):
Debt securities	19	17
Available-for-sale Securities	3	10
U.S. government and federal agency obligations	1	1
Total assets	1,513	1,886
Total liabilities	752	1,376
Fair Value, Measurements, Recurring \| Commodity contracts
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	745	1,199
Derivative liabilities	693	1,288
Fair Value, Measurements, Recurring \| Interest rate contracts
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	53	49
Derivative liabilities	59	88
Fair Value, Measurements, Recurring \| Cash and cash equivalents
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	47	25
Fair Value, Measurements, Recurring \| U.S. government and federal agency obligations
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	43	73
Fair Value, Measurements, Recurring \| Federal agency mortgage-backed securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	82	62
Fair Value, Measurements, Recurring \| Commercial mortgage-backed securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	14	17
Fair Value, Measurements, Recurring \| Corporate debt securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	99	84
Fair Value, Measurements, Recurring \| Equity securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	334	292
Investments, Fair Value Disclosure	68	54
Fair Value, Measurements, Recurring \| Foreign government fixed income securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	5	3
Fair Value, Measurements, Recurring \| Level 1
Investment in available-for-sale securities (classified within other non-current assets):
Debt securities	0	0
Available-for-sale Securities	3	10
U.S. government and federal agency obligations	1	1
Total assets	616	960
Total liabilities	257	494
Fair Value, Measurements, Recurring \| Level 1 \| Commodity contracts
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	191	560
Derivative liabilities	257	494
Fair Value, Measurements, Recurring \| Level 1 \| Interest rate contracts
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Cash and cash equivalents
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	45	25
Fair Value, Measurements, Recurring \| Level 1 \| U.S. government and federal agency obligations
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	42	72
Fair Value, Measurements, Recurring \| Level 1 \| Federal agency mortgage-backed securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Commercial mortgage-backed securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Corporate debt securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Equity securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	334	292
Fair Value, Measurements, Recurring \| Level 1 \| Foreign government fixed income securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 2
Investment in available-for-sale securities (classified within other non-current assets):
Debt securities	0	0
Available-for-sale Securities	0	0
U.S. government and federal agency obligations	0	0
Total assets	765	765
Total liabilities	418	724
Fair Value, Measurements, Recurring \| Level 2 \| Commodity contracts
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	509	549
Derivative liabilities	359	636
Fair Value, Measurements, Recurring \| Level 2 \| Interest rate contracts
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	53	49
Derivative liabilities	59	88
Fair Value, Measurements, Recurring \| Level 2 \| Cash and cash equivalents
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	2	0
Fair Value, Measurements, Recurring \| Level 2 \| U.S. government and federal agency obligations
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	1	1
Fair Value, Measurements, Recurring \| Level 2 \| Federal agency mortgage-backed securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	82	62
Fair Value, Measurements, Recurring \| Level 2 \| Commercial mortgage-backed securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	14	17
Fair Value, Measurements, Recurring \| Level 2 \| Corporate debt securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	99	84
Fair Value, Measurements, Recurring \| Level 2 \| Equity securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 2 \| Foreign government fixed income securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	5	3
Fair Value, Measurements, Recurring \| Level 3
Investment in available-for-sale securities (classified within other non-current assets):
Debt securities	19	17
Available-for-sale Securities	0	0
U.S. government and federal agency obligations	0	0
Total assets	64	107
Total liabilities	77	158
Fair Value, Measurements, Recurring \| Level 3 \| Commodity contracts
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	45	90
Derivative liabilities	77	158
Fair Value, Measurements, Recurring \| Level 3 \| Interest rate contracts
Investment in available-for-sale securities (classified within other non-current assets):
Derivative assets	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Cash and cash equivalents
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 3 \| U.S. government and federal agency obligations
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Federal agency mortgage-backed securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Commercial mortgage-backed securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Corporate debt securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Equity securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Foreign government fixed income securities
Investment in available-for-sale securities (classified within other non-current assets):
Nuclear trust fund investments	$ 0	$ 0

Discloses the fair value amount of decommissioning trust assets and other.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Total of debt securities categorized neither as held-to-maturity nor trading which are intended be sold or mature more than one year from the balance sheet date or operating cycle, if longer.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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The fair value as of the balance sheet date of firm holdings in debt obligations issued by the US government including short-term Treasury bills, medium-term Treasury notes, and long-term Treasury bonds, as well as debt issued by agencies. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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