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Long-term Debt (Details)
$ in Millions
9 Months Ended
Sep. 30, 2016
USD ($)
Aug. 08, 2016
USD ($)
May 13, 2016
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]        
Line of Credit Facility, Maximum Borrowing Capacity $ 300.0     $ 300.0
Debt Instrument, Term 5 years      
General mortgage bonds used as collateral $ 118.0     118.0
Revolving Credit Facility [Member]        
Debt Instrument [Line Items]        
Loans Outstanding 0.0     $ 200.0 [1]
Debt, Weighted Average Interest Rate       1.637%
Letter of Credit [Member]        
Debt Instrument [Line Items]        
Loans Outstanding $ 4.0     $ 4.0
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 0.517      
Percentage on limitation of debt to total capitalization under covenant amended 70.00%      
Restoration Cost Expected Cost $ 100.0      
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 $ 150.0      
June and July [Member] | Treasury Lock [Member]        
Debt Instrument [Line Items]        
Aggregate Notional Amount 300.0      
Bonds General Mortgage Due 2021 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount     $ 300.0  
Debt Instrument, Interest Rate, Stated Percentage     1.85%  
Bonds General Mortgage Due 2026 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount   $ 300.0    
Debt Instrument, Interest Rate, Stated Percentage   2.40%    
Cash Flow Hedging [Member]        
Debt Instrument [Line Items]        
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net $ 1.1      
[1] Weighted average interest rate was 1.637% as of December 31, 2015