Rate Matters Rate Matters (Details) - USD ($) $ in Millions	1 Months Ended
	May 31, 2018	Sep. 30, 2016	Jun. 30, 2016	Feb. 28, 2015	Nov. 30, 2013
Public Service Commission of Wisconsin (PSCW) [Member] | PSCW Proceeding - Tax Cuts and Jobs Act of 2017 [Member]
Public Utilities, General Disclosures [Line Items]
Tax Cuts and Jobs Act of 2017, Approved Refund of 2018 TCJA Benefits	$ 27
Tax Cuts and Jobs Act of 2017, Approved Deferral of 2018 TCJA Benefits	$ 5
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint
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 Regional Transmission Operator's Region, Recommended By Third Parties				8.67%	9.15%
NSP-Minnesota | Federal Energy Regulatory Commission (FERC) | FERC Proceeding, MISO ROE Complaint
Public Utilities, General Disclosures [Line Items]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Approved		10.32%
Public Utilities, Length of Refund Period, In Months		15 months
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, with RTO Adder, Approved		10.82%
Public Utilities, ROE Basis Point Adder, Approved		50
NSP-Minnesota | Administrative Law Judge | FERC Proceeding, MISO ROE Complaint
Public Utilities, General Disclosures [Line Items]
Public Utilities, ROE Applicable To Transmission Formula Rates In The Regional Transmission Operator's Region, Recommended By Third Parties			9.70%