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Long-Term Borrowings (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months $ 0.0  
Debt Instrument, Unamortized Discount (Premium), Net (7.0) $ (4.0)
Unamortized Debt Issuance Expense (23.0) (20.0)
Deferred Finance Costs, Noncurrent, Net 23.0 20.0
Long-term Debt, Maturities, Repayments of Principal in Year Two 0.0  
Long-term Debt, Maturities, Repayments of Principal in Year Three 0.0  
Long-term Debt, Maturities, Repayments of Principal in Year Four 0.0  
Long-term Debt, Maturities, Repayments of Principal in Year Five 350.0  
Long-term Debt, Excluding Current Maturities $ 2,419.7 2,126.1
First Mortgage Bonds, Series due: | Series Due June 15, 2024 [Member]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) 3.30%  
Face Amount $ 150.0 150.0
Long-term Debt, Excluding Current Maturities $ 2,420.0 2,126.0
First Mortgage Bonds, Series due: | Series Due June 15, 2024 2 [Member]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) 3.30%  
Face Amount $ 200.0 200.0
First Mortgage Bonds, Series due: | Series Due Aug. 15, 2041 [Member]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) 4.50%  
Face Amount $ 200.0 200.0
First Mortgage Bonds, Series due: | Series Due Aug. 15, 2041 2 [Member]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) 4.50%  
Face Amount $ 100.0 100.0
First Mortgage Bonds, Series due: | Series Due Aug. 15, 2041 3 [Member]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) 4.50%  
Face Amount $ 100.0 100.0
First Mortgage Bonds, Series due: | Series Due August 15, 2046 [Member]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) 3.40%  
Face Amount $ 300.0 300.0
First Mortgage Bonds, Series due: | Series Due August 15, 2047 [Member]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) 3.70%  
Face Amount $ 450.0 450.0
First Mortgage Bonds, Series due: | Series Due Nov. 15, 2048 [Member]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) [1] 4.40%  
Face Amount [1] $ 300.0 300.0
First Mortgage Bonds, Series due: | Series Due June 15, 2049 [Domain]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) [2] 3.75%  
Face Amount [2] $ 300.0 0.0
Unsecured Debt [Member] | Senior C and D Due Oct. 1, 2033 [Member]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) 6.00%  
Face Amount $ 100.0 100.0
Unsecured Debt [Member] | Senior F Due Oct. 1, 2036 [Member]    
Debt Instrument [Line Items]    
Interest Rate, Stated Percentage (in hundredths) 6.00%  
Face Amount $ 250.0 $ 250.0
[1]
2018 financing
[2]
2019 financing