Balance Sheet Components (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Jun. 30, 2016	May 04, 2016
Valuation and Qualifying Accounts Disclosure [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	$ 265,000,000		$ 257,000,000
Amortization of Intangible Assets		$ 11,000,000
Phase-in Plan, Amount of Costs Deferred for Rate-making Purposes	514,000,000
Syndicated credit facility scheduled to expire in 2019	$ 500,000,000
Debt Instrument, Covenant Description	0.65
Ratio of Indebtedness to Net Capital	0.506
Short-term debt	$ 0
Line of Credit Facility, Remaining Borrowing Capacity	500,000,000
Line of Credit Facility, Current Borrowing Capacity	160,000,000
Letters of credit issued	56,000,000
Authorized Short-Term Debt	$ 900,000,000
Debt Instrument, Redemption, Description	200
Proceeds from Issuance of Long-term Debt			$ 25,000,000	$ 75,000,000	$ 50,000,000
Unsecured term bank loan rate - minimum	1.60%
Extinguishment of Debt, Amount	$ 0	$ 133,000,000