Balance Sheet Components (Details)	3 Months Ended			9 Months Ended
	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Asset Retirement Obligation, Revision of Estimate		$ 23,000,000
Letters of Credit Outstanding, Amount	$ 0			$ 0
Finite-Lived Intangible Assets, Accumulated Amortization	333,000,000			333,000,000		$ 296,000,000
Amortization of Intangible Assets	16,000,000		$ 11,000,000	43,000,000	$ 34,000,000
Syndicated credit facility scheduled to expire in 2019	$ 500,000,000			$ 500,000,000
Debt Instrument, Covenant Description				0.65
Ratio of Indebtedness to Net Capital	0.512			0.512
Short-term debt	$ 0			$ 0
Commercial Paper	0			0
Line of Credit Facility, Remaining Borrowing Capacity	500,000,000			500,000,000
Line of Credit Facility, Current Borrowing Capacity	220,000,000			220,000,000
Letters of credit issued	59,000,000			59,000,000
Authorized Short-Term Debt	900,000,000			900,000,000
Long-term Debt, Current Maturities	300,000,000			300,000,000		$ 0
Long-term Pollution Control Bond	24,000,000			$ 24,000,000
Extinguishment of Debt, Amount	$ 24,000,000