Commitments and Contingencies - MISO ROE Complaints (Details) - Federal Energy Regulatory Commission (FERC) - FERC Proceeding, MISO ROE Complaint - NSP Minnesota and NSP Wisconsin [Member] [Member]	1 Months Ended		3 Months Ended	7 Months Ended	10 Months Ended	11 Months Ended	17 Months Ended
	Feb. 28, 2015	Nov. 30, 2013	Dec. 31, 2016	Dec. 31, 2020	Mar. 31, 2021	Mar. 31, 2021	Mar. 31, 2021
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 developed with new approach							9.88%
Public Utilities, ROE New Base, Complaint Number 1						10.02%
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%