Balance Sheet Components (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	May 04, 2016
Valuation and Qualifying Accounts Disclosure [Line Items]
Letters of Credit Outstanding, Amount	$ 0		$ 0
Finite-Lived Intangible Assets, Accumulated Amortization	277,000,000		277,000,000		$ 257,000,000
Amortization of Intangible Assets	12,000,000	$ 10,000,000	23,000,000	$ 22,000,000
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment	635,000,000		635,000,000
Phase-in Plan, Amount of Costs Deferred for Rate-making Purposes	514,000,000		514,000,000
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	0.510		0.510
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	56,000,000		56,000,000
Authorized Short-Term Debt	$ 900,000,000		$ 900,000,000
Debt Instrument, Redemption, Description	25		200
Proceeds from Issuance of Long-term Debt	$ 75,000,000		$ 75,000,000			$ 50,000,000
Debt Instrument, Interest Rate, Stated Percentage	0.00%		0.00%
Extinguishment of Debt, Amount			$ 0	$ 133,000,000