Balance Sheet Components (Details)	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Valuation and Qualifying Accounts Disclosure [Line Items]
Asset Retirement Obligation, Revision of Estimate	$ 23,000,000
Letters of Credit Outstanding, Amount	0
Finite-Lived Intangible Assets, Accumulated Amortization	303,000,000		$ 296,000,000
Amortization of Intangible Assets	13,000,000	$ 11,000,000
Syndicated credit facility scheduled to expire in 2019	$ 500,000,000
Debt Instrument, Covenant Description	0.65
Ratio of Indebtedness to Net Capital	0.514
Short-term debt	$ 0
Commercial Paper	0
Line of Credit Facility, Remaining Borrowing Capacity	500,000,000
Line of Credit Facility, Current Borrowing Capacity	220,000,000
Letters of credit issued	71,000,000
Authorized Short-Term Debt	$ 900,000,000