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Commitments and Contingencies, Legal Contingencies (Details)
1 Months Ended 12 Months Ended
Jan. 31, 2019
USD ($)
Nov. 30, 2017
Sep. 30, 2016
Dec. 31, 2015
Feb. 28, 2015
Nov. 30, 2013
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Gas Trading Litigation                  
Legal Contingencies [Abstract]                  
Loss Contingency, Pending Claims, Number             2    
Loss Contingency, Settled Claims, number             4    
NSP-Minnesota                  
Legal Contingencies [Abstract]                  
Percentage of Fault             48.00%    
NSP-Minnesota | Insurance-related Assessments [Member]                  
Loss Contingencies [Line Items]                  
Nuclear Insurance Coverage Exposure Funded By Secondary Financial Protection Program             $ 13,600,000,000    
Legal Contingencies [Abstract]                  
Maximum Assessments Per Reactor Per Accident             $ 138,000,000    
PSCo | Line Extension Disputes                  
Legal Contingencies [Abstract]                  
Accrual For Legal Contingency               $ 0  
General Electric (GE) [Domain]                  
Legal Contingencies [Abstract]                  
Percentage of Fault             52.00%    
Minimum | PSCo | Line Extension Disputes                  
Legal Contingencies [Abstract]                  
Loss Contingency, Number of Plaintiffs       50          
Summary Judgment Granted Against Plaintiff [Member] | Gas Trading Litigation                  
Legal Contingencies [Abstract]                  
Loss Contingency, Number of Plaintiffs   2              
Remaining in Litigation [Member] | Gas Trading Litigation                  
Legal Contingencies [Abstract]                  
Loss Contingency, Number of Plaintiffs   3              
FERC Proceeding, MISO ROE Complaint [Member] | NSP-Minnesota                  
Loss Contingencies [Line Items]                  
Public Utilities, Number of steps required             2    
Legal Contingencies [Abstract]                  
Public Utilities, Base Return On Equity Charged To Customers Through Transmission Formula Rates         12.38% 12.38%      
Public Utilities, ROE developed with new approach             10.28%    
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties         8.67% 9.15%      
Federal Energy Regulatory Commission (FERC) [Member] | FERC Proceeding, MISO ROE Complaint [Member] | NSP-Minnesota                  
Legal Contingencies [Abstract]                  
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Approved     10.32% 10.32%          
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, with RTO Adder, Approved               10.82%  
Southwest Power Pool (SPP) [Member] | SPP Open Access Transmission Tariff Upgrade Costs [Member] | SPS                  
Legal Contingencies [Abstract]                  
Public Utilities, Billed Charges For Transmission Service Upgrades                 $ 13,000,000
Subsequent Event | NSP-Minnesota                  
Legal Contingencies [Abstract]                  
Customer refund of previously recovered purchased power costs $ 20,000,000