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Long-term Debt (Details)
$ in Millions
3 Months Ended
Mar. 31, 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 [1] $ 200 $ 200
Debt, Weighted Average Interest Rate 1.56% 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 0.526  
Percentage on limitation of debt to total capitalization under covenant amended (in hundredths) 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%  
Treasury Lock [Member]    
Debt Instrument [Line Items]    
Aggregate Notional Amount $ 150  
[1] Weighted average interest rate was 1.560% and 1.637% as of March 31, 2016 and December 31, 2015, respectively.