Balance Sheet Components (Details)		3 Months Ended		6 Months Ended
	Apr. 12, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 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		334,000,000		334,000,000		$ 302,000,000
Amortization of Intangible Assets		$ 17,000,000	$ 14,000,000	$ 33,000,000	$ 27,000,000
Line of Credit Facility, Maximum Borrowing Capacity						500,000,000
Debt Instrument, Covenant Description				65.00%
Ratio of Indebtedness to Net Capital		0.503		0.503
Short-term debt		$ 17,000,000		$ 17,000,000		0
Commercial Paper		17,000,000		17,000,000
Line of Credit Facility, Remaining Borrowing Capacity		483,000,000		483,000,000
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		0		0		$ 300,000,000
Proceeds from Issuance of Long-term Debt	$ 200,000,000			200,000,000	$ 0
Letter of Credit [Member]
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Short-term debt		$ 0		$ 0