Commitments and Contingencies FERC NOPR on ROE Incentive Adders (Details) - Federal Energy Regulatory Commission (FERC) [Member] - NSP Minnesota, NSP Wisconsin and SPS	Mar. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
ROE incentive adders related to FERC NOPR	50.00%
Public Utilities, General Disclosures [Line Items]
ROE incentive adders related to FERC NOPR	50.00%

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