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Long-term Debt (Details)
$ in Millions
6 Months Ended
Jun. 30, 2016
USD ($)
May 13, 2016
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity $ 300   $ 300
Debt Instrument, Term 5 years    
General mortgage bonds used as collateral $ 118   118
Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Loans Outstanding 0   $ 200 [1]
Debt, Weighted Average Interest Rate     1.637%
Letter of Credit [Member]      
Debt Instrument [Line Items]      
Loans Outstanding $ 4   $ 4
Line of Credit [Member]      
Debt Instrument [Line Items]      
Percentage on limitation of debt to total capitalization under covenant 65.00%    
Ratio of Indebtedness to Net Capital 0.527    
Percentage on limitation of debt to total capitalization under covenant amended 70.00%    
Restoration Cost Expected Cost $ 100    
Consecutive Period for System Restoration Costs to Exceed $100 million (in months) 12    
Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Interest Rate Description 1.125%    
April [Member] | Treasury Lock [Member]      
Debt Instrument [Line Items]      
Aggregate Notional Amount $ 150    
June and July [Member] | Treasury Lock [Member]      
Debt Instrument [Line Items]      
Aggregate Notional Amount 300    
Bonds General Mortgage Due 2021 [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount   $ 300  
Debt Instrument, Interest Rate, Stated Percentage   1.85%  
Current Liabilities [Member] | Interest Rate Contract [Member]      
Debt Instrument [Line Items]      
Interest Rate Derivative Liabilities, at Fair Value $ 2    
[1] Weighted average interest rate was 1.637% as of December 31, 2015