Fair Value Measurements Recurring Measurements and Additional (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 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
Carrying Amount [Member]
Additional disclosures:
Other receivables	7	$ 7
Long-term debt, including current portion	(17,512)	(16,497)
Fair Value [Member]
Additional disclosures:
Other receivables	7	7
Long-term debt, including current portion	(18,307)	(18,112)
Level 1 [Member]
Additional disclosures:
Other receivables	7	7
Long-term debt, including current portion	0	0
Level 2 [Member]
Additional disclosures:
Other receivables	0	0
Long-term debt, including current portion	(18,307)	(18,112)
Level 3 [Member]
Additional disclosures:
Other receivables	0	0
Long-term debt, including current portion	0	0
Fair Value, Measurements, Recurring [Member] | Carrying Amount [Member]
Measured on a recurring basis:
ARO Trust investments	145	135
Fair Value, Measurements, Recurring [Member] | Carrying Amount [Member] | Designated as Hedging Instrument [Member]
Measured on a recurring basis:
Energy derivatives assets	2
Energy derivatives liabilities	(3)	(3)
Fair Value, Measurements, Recurring [Member] | Carrying Amount [Member] | Not Designated as Hedging Instrument [Member]
Measured on a recurring basis:
Energy derivatives assets	4
Energy derivatives liabilities	(4)	(3)
Fair Value, Measurements, Recurring [Member] | Fair Value [Member]
Measured on a recurring basis:
ARO Trust investments	145	135
Fair Value, Measurements, Recurring [Member] | Fair Value [Member] | Designated as Hedging Instrument [Member]
Measured on a recurring basis:
Energy derivatives assets	2
Energy derivatives liabilities	(3)	(3)
Fair Value, Measurements, Recurring [Member] | Fair Value [Member] | Not Designated as Hedging Instrument [Member]
Measured on a recurring basis:
Energy derivatives assets	4
Energy derivatives liabilities	(4)	(3)
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Measured on a recurring basis:
ARO Trust investments	145	135
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Designated as Hedging Instrument [Member]
Measured on a recurring basis:
Energy derivatives assets	2
Energy derivatives liabilities	(3)	(2)
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Not Designated as Hedging Instrument [Member]
Measured on a recurring basis:
Energy derivatives assets	4
Energy derivatives liabilities	(1)	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 assets	0
Energy derivatives liabilities	0	(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 assets	0
Energy derivatives liabilities	0	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)