Balance Sheet Components (Details)				3 Months Ended		9 Months Ended
	Nov. 15, 2019 USD ($)	Oct. 25, 2019 USD ($)	Apr. 12, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Letters of Credit Outstanding, Amount				$ 0		$ 0
Finite-Lived Intangible Assets, Accumulated Amortization				350,000,000		350,000,000		$ 302,000,000
Amortization of Intangible Assets				16,000,000	$ 16,000,000	$ 49,000,000	$ 43,000,000
Asset Retirement Obligation, Revision of Estimate				$ 69,000,000
Line of Credit Facility, Maximum Borrowing Capacity								500,000,000
Debt Instrument, Covenant Description						65.00%
Ratio of Indebtedness to Net Capital				0.502		0.502
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				60,000,000		60,000,000
Authorized Short-Term Debt				900,000,000		900,000,000
Long-term Debt, Current Maturities				50,000,000		50,000,000		$ 300,000,000
Proceeds from Issuance of Debt		$ 270,000,000
Proceeds from Issuance of Long-term Debt	$ 160,000,000	$ 110,000,000	$ 200,000,000			200,000,000	$ 0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment				2,000,000
Letter of Credit [Member]
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Short-term debt				$ 0		$ 0