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Balance Sheet Components (Details) - USD ($)
 3 Months Ended 9 Months Ended
Jan. 09, 2016
Sep. 30, 2016
Mar. 31, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
May 04, 2016
Jan. 31, 2016
Jan. 11, 2016
Jan. 08, 2016
Dec. 31, 2015
Valuation and Qualifying Accounts Disclosure [Line Items]                      
Finite-Lived Intangible Assets, Accumulated Amortization   $ 260,000,000     $ 260,000,000           $ 227,000,000
Amortization of Intangible Assets   11,000,000   $ 10,000,000 33,000,000 $ 28,000,000          
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment   615,000,000     615,000,000           424,000,000
Phase-in Plan, Amount of Costs Deferred for Rate-making Purposes   514,000,000     514,000,000            
Capital Leased Assets, Gross   57,000,000     57,000,000            
Amortization of Leased Asset   1,000,000     2,000,000            
Capital Leases, Income Statement, Interest Expense   1,000,000     3,000,000            
Capital Leases, Future Minimum Payments, Remainder of Fiscal Year   2,000,000     2,000,000            
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           $ 6,000,000
Line of Credit Facility, Remaining Borrowing Capacity   500,000,000     500,000,000            
Line of Credit Facility, Current Borrowing Capacity   160,000,000     160,000,000            
Letters of credit issued   73,000,000     73,000,000            
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     $ 140,000,000  
Unsecured term bank loan rate - minimum   1.20%     1.20%            
Debt Instrument, Interest Rate, Stated Percentage               3.81% 5.80% 2.51%  
Extinguishment of Debt, Amount     $ 133,000,000   $ 133,000,000 $ 442,000,000          
Notes Payable to Banks [Member]                      
Valuation and Qualifying Accounts Disclosure [Line Items]                      
Extinguishment of Debt, Amount $ 75,000,000                    
Long-term Debt [Member]                      
Valuation and Qualifying Accounts Disclosure [Line Items]                      
Extinguishment of Debt, Amount $ 58,000,000