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Commitments and Contingencies MISO ROE Complaints (Details) - NSP Minnesota and NSP Wisconsin - FERC Proceeding, MISO ROE Complaint [Member] - Federal Energy Regulatory Commission (FERC) [Member]
 1 Months Ended 3 Months Ended 7 Months Ended 16 Months Ended 17 Months Ended 23 Months Ended
Feb. 28, 2015
Nov. 30, 2013
Jun. 30, 2017
Dec. 31, 2020
Sep. 30, 2021
Sep. 30, 2021
Sep. 30, 2021
Public Utilities, General Disclosures [Line Items]              
Public Utilities, Base Return On Equity Charged To Customers Through Transmission Formula Rates 1238.00% 1238.00%          
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties 867.00% 915.00%          
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Approved     1032.00%        
Public Utilities, ROE developed with new approach             988.00%
Public Utilities, ROE New Base, Complaint Number 1           1002.00%  
Basis Point Reduction         1000.00%    
Basis Point Reduction - First Complaint       100000000.00%      
Basis Point Reduction - Second Complaint       100000000.00%      
Basis Point Reduction - Third Complaint       200000000.00%