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Borrowings and Other Financing Instruments Long-Term Borrowings (Details) - Bonds [Member] - USD ($)
Jul. 19, 2021
Mar. 23, 2021
Mar. 02, 2021
Mar. 01, 2021
PSCo | Series Due June 15, 2031        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount       $ 750,000,000
Debt Instrument, Interest Rate, Stated Percentage       1.875%
SPS | Series Due 2050        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount     $ 250,000,000  
Debt Instrument, Interest Rate, Stated Percentage     3.15%  
NSP-Wisconsin | Series Due May 1, 2051 [Domain]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount $ 100,000,000      
Debt Instrument, Interest Rate, Stated Percentage 2.82%      
NSP-Minnesota | Series Due April 1, 2031        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount   $ 425,000,000    
Debt Instrument, Interest Rate, Stated Percentage   2.25%    
NSP-Minnesota | Series Due April 1, 2052        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount   $ 425,000,000    
Debt Instrument, Interest Rate, Stated Percentage   3.20%