Fair Value Measurements Recurring Measurements and Additional (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Additional disclosures:
Fair Value, Level 1 to level 2 Transfers, Amount	$ 0	$ 0
Fair Value, Level 2 to level 1 Transfers, Amount	0	0
Wiltel Guarantee [Member]
Additional disclosures:
Guarantor Obligations, Maximum Exposure, Undiscounted	29
Carrying Amount [Member]
Additional disclosures:
Long-term debt, including current portion	(22,414)	(20,935)
Guarantees	(43)	(43)
Carrying Amount [Member] \| Indemnification Agreement [Member]
Additional disclosures:
Guarantees	0
Fair Value [Member]
Additional disclosures:
Long-term debt, including current portion	(23,330)	(23,005)
Guarantees	(30)	(30)
Level 1 [Member]
Additional disclosures:
Long-term debt, including current portion	0	0
Guarantees	0	0
Level 2 [Member]
Additional disclosures:
Long-term debt, including current portion	(23,330)	(23,005)
Guarantees	(14)	(14)
Level 3 [Member]
Additional disclosures:
Long-term debt, including current portion	0	0
Guarantees	(16)	(16)
Fair Value, Measurements, Recurring [Member] \| Carrying Amount [Member]
Measured on a recurring basis
ARO Trust investments	150	135
Fair Value, Measurements, Recurring [Member] \| Carrying Amount [Member] \| Designated as Hedging Instrument [Member]
Measured on a recurring basis
Energy derivatives liabilities		(3)
Fair Value, Measurements, Recurring [Member] \| Carrying Amount [Member] \| Not Designated as Hedging Instrument [Member]
Measured on a recurring basis
Energy derivatives assets	3
Energy derivatives liabilities	(7)	(3)
Fair Value, Measurements, Recurring [Member] \| Fair Value [Member]
Measured on a recurring basis
ARO Trust investments	150	135
Fair Value, Measurements, Recurring [Member] \| Fair Value [Member] \| Designated as Hedging Instrument [Member]
Measured on a recurring basis
Energy derivatives liabilities		(3)
Fair Value, Measurements, Recurring [Member] \| Fair Value [Member] \| Not Designated as Hedging Instrument [Member]
Measured on a recurring basis
Energy derivatives assets	3
Energy derivatives liabilities	(7)	(3)
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Measured on a recurring basis
ARO Trust investments	150	135
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Designated as Hedging Instrument [Member]
Measured on a recurring basis
Energy derivatives liabilities		(2)
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Not Designated as Hedging Instrument [Member]
Measured on a recurring basis
Energy derivatives assets	3
Energy derivatives liabilities	(4)	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Measured on a recurring basis
ARO Trust investments	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Designated as Hedging Instrument [Member]
Measured on a recurring basis
Energy derivatives liabilities		(1)
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Not Designated as Hedging Instrument [Member]
Measured on a recurring basis
Energy derivatives assets	0
Energy derivatives liabilities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Measured on a recurring basis
ARO Trust investments	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Designated as Hedging Instrument [Member]
Measured on a recurring basis
Energy derivatives liabilities		0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Not Designated as Hedging Instrument [Member]
Measured on a recurring basis
Energy derivatives assets	0
Energy derivatives liabilities	$ (3)	$ (3)

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Carrying amount of an asset that is legally restricted for purposes of settling an asset retirement obligation.

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Fair value, before effects of master netting arrangements, of a financial asset or other 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 assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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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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- References

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

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Fair value portion of guarantee, including, but not limited to, financial standby letter of credit, fair value guarantee, guarantee of collection of scheduled contractual cash flows from financial assets, and contracts that contingently require the guarantor to make payments or perform services.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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