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Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Long-Term Borrowings [Line Items]    
Long-term Line of Credit   $ 0
Deferred Finance Costs, Noncurrent, Net $ 26,600,000 26,500,000
First Mortgage Bonds | Series Due May 15, 2025 [Member]    
Long-Term Borrowings [Line Items]    
Interest rate, stated percentage (in hundredths) 2.90%  
Maturity date May 15, 2025  
Long-term Debt, Gross $ 250,000,000 0
First Mortgage Bonds | Series Due March 15, 2044    
Long-Term Borrowings [Line Items]    
Face amount   $ 300,000,000
Interest rate, stated percentage (in hundredths) 4.30% 4.30%
Maturity date Mar. 15, 2044 Mar. 15, 2044
Long-term Debt, Gross $ 300,000,000 $ 300,000,000
First Mortgage Bonds | Series Due Sept. 1, 2017    
Long-Term Borrowings [Line Items]    
Interest rate, stated percentage (in hundredths) 4.375% 4.375%
Maturity date Sep. 01, 2017 Sep. 01, 2017
Long-term Debt, Gross [1] $ 129,500,000 $ 129,500,000
First Mortgage Bonds | Series Due Aug. 1, 2018    
Long-Term Borrowings [Line Items]    
Interest rate, stated percentage (in hundredths) 5.80% 5.80%
Maturity date Aug. 01, 2018 Aug. 01, 2018
Long-term Debt, Gross $ 300,000,000 $ 300,000,000
First Mortgage Bonds | Series Due June 1, 2019    
Long-Term Borrowings [Line Items]    
Interest rate, stated percentage (in hundredths) 5.125% 5.125%
Maturity date Jun. 01, 2019 Jun. 01, 2019
Long-term Debt, Gross $ 400,000,000 $ 400,000,000
First Mortgage Bonds | Series Due Nov. 15, 2020    
Long-Term Borrowings [Line Items]    
Interest rate, stated percentage (in hundredths) 3.20% 3.20%
Maturity date Nov. 15, 2020 Nov. 15, 2020
Long-term Debt, Gross $ 400,000,000 $ 400,000,000
PSCo [Member] | First Mortgage Bonds | Series Due May 15, 2025 [Member]    
Long-Term Borrowings [Line Items]    
Face amount $ 250,000,000  
Interest rate, stated percentage (in hundredths) 2.90%  
[1] Pollution control financing.