Commitments and Contingencies MISO ROE Complaints (Details) - Federal Energy Regulatory Commission (FERC) [Member] - NSP Minnesota and NSP Wisconsin [Member] [Member] - FERC Proceeding, MISO ROE Complaint [Member]	1 Months Ended		7 Months Ended	8 Months Ended	9 Months Ended
	Feb. 28, 2015	Nov. 30, 2013	Dec. 31, 2020	Dec. 31, 2020	Sep. 30, 2018
Public Utilities, General Disclosures [Line Items]
Public Utilities, Base Return On Equity Charged To Customers Through Transmission Formula Rates	12.38%	12.38%
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties	8.67%	9.15%
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Approved					10.32%
Public Utilities, ROE New Base, Complaint Number 1				10.02%
Basis Point Reduction			1000.00%
Basis Point Reduction - First Complaint			200000000.00%
Basis Point Reduction - Second Complaint			100000000.00%