Balance Sheet Components (Details) - USD ($)		3 Months Ended				9 Months Ended
	May. 22, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	May. 21, 2015	May. 19, 2015	Jan. 31, 2015	Dec. 31, 2014
Valuation and Qualifying Accounts Disclosure [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization		$ 219,000,000				$ 219,000,000					$ 191,000,000
Amortization of Intangible Assets		10,000,000			$ 6,000,000	28,000,000	$ 18,000,000
Asset Retirement Obligations, Significant Changes			15
Line of Credit Facility, Current Borrowing Capacity		135,000,000		$ 500,000,000		135,000,000					$ 700,000,000
Syndicated credit facility scheduled to expire in 2019		$ 500,000,000				$ 500,000,000
Debt Instrument, Covenant Description						.65
Ratio of Indebtedness to Net Capital		0.5				0.5
Short-term Debt		$ 0				$ 0
Borrowings		3,000,000				3,000,000
Line of Credit Facility, Remaining Borrowing Capacity		497,000,000				497,000,000
Letters of credit issued		93,000,000				93,000,000
Authorized Short-Term Debt		$ 900,000,000				$ 900,000,000
Debt Instrument, Interest Rate, Stated Percentage		3.46%				3.46%		6.80%	3.50%	3.55%
Extinguishment of Debt, Amount						$ 442,000,000	$ 0
[Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Proceeds from Issuance of Long-term Debt				75,000,000					$ 70,000,000
Notes Payable to Banks [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Extinguishment of Debt, Amount	$ 67,000,000	$ 55,000,000	$ 200,000,000	$ 50,000,000
Long-term Debt [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Extinguishment of Debt, Amount						$ 70,000,000