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Long-Term Debt (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Long-Term Borrowings and Other Financing Instruments    
Long-term Debt, Gross $ 5,521.3 $ 4,937.2
Unamortized discount (31.0) (21.0)
Unamortized debt expense (48.0) (42.0)
Long-term Debt, Current Maturities 300.0 0.0
Long-term Debt 5,221.0 4,937.0
2020 300.0  
2021 0.0  
2022 300.0  
2023 400.0  
2024 0.0  
First Mortgage Bonds | Series Due Aug. 15, 2020 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 300.0 300.0
Debt Instrument, Interest Rate, Stated Percentage 2.20%  
First Mortgage Bonds | Series Due Aug. 15, 2022 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 300.0 300.0
Debt Instrument, Interest Rate, Stated Percentage 2.15%  
First Mortgage Bonds | Series Due May 15, 2023 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 400.0 400.0
Debt Instrument, Interest Rate, Stated Percentage 2.60%  
First Mortgage Bonds | Series Due July 1, 2025 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 250.0 250.0
Debt Instrument, Interest Rate, Stated Percentage 7.13%  
First Mortgage Bonds | Series Due March 1, 2028 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 150.0 150.0
Debt Instrument, Interest Rate, Stated Percentage 6.50%  
First Mortgage Bonds | Series Due July 15, 2035 [Domain]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 250.0 250.0
Debt Instrument, Interest Rate, Stated Percentage 5.25%  
First Mortgage Bonds | Series Due June 1, 2036 [Domain]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 400.0 400.0
Debt Instrument, Interest Rate, Stated Percentage 6.25%  
First Mortgage Bonds | Series Due July 1, 2037 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 350.0 350.0
Debt Instrument, Interest Rate, Stated Percentage 6.20%  
First Mortgage Bonds | Series Due Nov. 1, 2039 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 300.0 300.0
Debt Instrument, Interest Rate, Stated Percentage 5.35%  
First Mortgage Bonds | Series Due Aug. 15, 2040 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 250.0 250.0
Debt Instrument, Interest Rate, Stated Percentage 4.85%  
First Mortgage Bonds | Series Due Aug. 15, 2042 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 500.0 500.0
Debt Instrument, Interest Rate, Stated Percentage 3.40%  
First Mortgage Bonds | Series Due May 15, 2044 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 300.0 300.0
Debt Instrument, Interest Rate, Stated Percentage 4.13%  
First Mortgage Bonds | Series Due Aug. 15, 2045 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 300.0 300.0
Debt Instrument, Interest Rate, Stated Percentage 4.00%  
First Mortgage Bonds | Series Due May 15, 2046 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 350.0 350.0
Debt Instrument, Interest Rate, Stated Percentage 3.60%  
First Mortgage Bonds | Series Due Sept. 15, 2047 [Member]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount $ 600.0 600.0
Debt Instrument, Interest Rate, Stated Percentage 3.60%  
First Mortgage Bonds | Series Due March 1, 2050 [Domain]    
Long-Term Borrowings and Other Financing Instruments    
Face Amount [1] $ 600.0 $ 0.0
Debt Instrument, Interest Rate, Stated Percentage [1] 2.90%  
Letter of Credit [Member] | Letter of Credit [Member]    
Long-Term Borrowings and Other Financing Instruments    
Line of Credit Facility, Expiration Period 1 year  
[1]
2019 financing