Balance Sheet Components (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Dec. 31, 2016
Valuation and Qualifying Accounts Disclosure [Line Items]
Letters of Credit Outstanding, Amount	$ 0		$ 0
Finite-Lived Intangible Assets, Accumulated Amortization	288,000,000		288,000,000			$ 257,000,000
Amortization of Intangible Assets	11,000,000	$ 11,000,000	34,000,000	$ 33,000,000
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	0.513		0.513
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	54,000,000		54,000,000
Authorized Short-Term Debt	900,000,000		900,000,000
Proceeds from Issuance of Long-term Debt	$ 225,000,000		$ 225,000,000		$ 150,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.98%		3.98%
Other Notes Payable	$ 75,000,000		$ 75,000,000
Notes Payable	150,000,000		150,000,000
Debt Instrument, Redemption, Description	200,000,000		200,000,000
Extinguishment of Debt, Amount	50,000,000		$ 50,000,000	$ 133,000,000
Debt Instrument, Interest Rate, Increase (Decrease)			6300.00%
Debt Instrument, Fee Amount	$ 0		$ 0