Rate Matters Rate Matters (Details) - USD ($) $ in Millions	1 Months Ended							9 Months Ended
	Sep. 30, 2017	May 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Feb. 28, 2015	Nov. 30, 2013	Sep. 30, 2017
NSP-Wisconsin | Public Service Commission of Wisconsin (PSCW) | PSCW Proceeding - Wisconsin 2018 Electric and Natural Gas Rate Case - Electric Rates 2018 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 24.7
Public Utilities, Requested Rate Increase (Decrease), Percentage		3.60%
Public Utilities, Requested Rate Base, Amount		$ 1,200.0
NSP-Wisconsin | Public Service Commission of Wisconsin (PSCW) | PSCW Proceeding - Wisconsin 2018 Electric and Natural Gas Rate Case - Gas Rates 2018 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 12.0
Public Utilities, Requested Rate Increase (Decrease), Percentage		10.10%
Public Utilities, Requested Rate Base, Amount		$ 138.4
NSP-Wisconsin | Public Service Commission of Wisconsin (PSCW) | PSCW Proceeding - Wisconsin 2018 Electric and Natural Gas Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Return on Equity, Percentage		10.00%
Public Utilities, Requested Equity Capital Structure, Percentage		52.53%
NSP-Wisconsin | Public Service Commission of Wisconsin (PSCW) Staff | PSCW Proceeding - Wisconsin 2018 Electric and Natural Gas Rate Case - Electric Rates 2018 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 10.9
Public Utilities, Requested Rate Increase (Decrease), Percentage	1.60%
NSP-Wisconsin | Public Service Commission of Wisconsin (PSCW) Staff | PSCW Proceeding - Wisconsin 2018 Electric and Natural Gas Rate Case - Gas Rates 2018 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 9.9
Public Utilities, Requested Rate Increase (Decrease), Percentage	8.30%
NSP-Wisconsin | Public Service Commission of Wisconsin (PSCW) Staff | PSCW Proceeding - Wisconsin 2018 Electric and Natural Gas Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Return on Equity, Percentage	9.80%
Public Utilities, Requested Equity Capital Structure, Percentage	51.45%
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%		10.32%
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended					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%