Rate Matters Rate Matters (Details) $ in Thousands				1 Months Ended					3 Months Ended		12 Months Ended
		Apr. 01, 2016 USD ($)	Jan. 06, 2015	Dec. 31, 2015	Mar. 31, 2015 USD ($)	Feb. 28, 2015	Jun. 30, 2014	Nov. 30, 2013	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 MW	Dec. 31, 2013 USD ($)	Dec. 31, 2008 USD ($)
Public Utilities, General Disclosures [Line Items]
Loss on Monticello life cycle management/extended power uprate project									$ 0	$ 5,237
NSP-Minnesota | MPUC Proceeding - Nuclear Project Prudency Investigation
Public Utilities, General Disclosures [Line Items]
Nuclear Project Expenditures, Amount												$ 665,000
Total Capitalized Nuclear Project Costs												$ 748,000
Initial Estimated Nuclear Project Expenditures													$ 320,000
NSP-Minnesota | MPUC Proceeding - Nuclear Project Prudency Investigation | Minnesota Public Utilities Commission [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Amount Of Recoverable Investment, With Return					$ 415,000
Public Utilities, Amount Of Recoverable Investment, Without A Return					$ 333,000
Public Utilities, Percentage Of Investment Considered Used And Useful					50.00%
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member]
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%
Public Utilities, Maximum Equity Capital Structure Percentage Allowed Per The Complaint								50.00%
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member] | Federal Energy Regulatory Commission (FERC)
Public Utilities, General Disclosures [Line Items]
Public Utilities, Number Of Steps Required For Newly Adopted ROE Discounted Cash Flow Methodology							2
Public Utilities, ROE Basis Point Adder Requested By Third Parties			50
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member] | Administrative Law Judge [Member]
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				10.32%
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member] | MPUC, NDPSC, SDPUC, and DOC [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Upper Bound, Percentage									8.82%
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Lower Bound, Percentage									8.81%
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member] | FERC Staff [Member]
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									8.78%
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member] | MISO TOs [Member]
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									10.92%
Xcel Energy Inc. | MPUC Proceeding - Nuclear Project Prudency Investigation
Public Utilities, General Disclosures [Line Items]
Loss on Monticello life cycle management/extended power uprate project										129,000
NSP-Wisconsin | MPUC Proceeding - Nuclear Project Prudency Investigation
Public Utilities, General Disclosures [Line Items]
Loss on Monticello life cycle management/extended power uprate project										$ 5,000
Minimum | NSP-Minnesota | MPUC Proceeding - Nuclear Project Prudency Investigation
Public Utilities, General Disclosures [Line Items]
Public Utilities, Facility Generating Capacity, In MW | MW											600
Minimum | NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Decrease In Transmission Revenue, Net Of Expense, Due To New ROE Methodology									$ 8,000
Maximum | NSP-Minnesota | MPUC Proceeding - Nuclear Project Prudency Investigation
Public Utilities, General Disclosures [Line Items]
Public Utilities, Facility Generating Capacity, In MW | MW											671
Maximum | NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Decrease In Transmission Revenue, Net Of Expense, Due To New ROE Methodology									$ 10,000
Subsequent Event [Member] | NSP-Wisconsin | PSCW Proceeding - Wisconsin 2017 Electric and Gas Rate Case - Electric Rates 2017 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 17,400
Public Utilities, Requested Rate Increase (Decrease), Percentage		2.40%
Public Utilities, Requested Rate Base, Amount		$ 1,188,000
Public Utilities, Requested Increase Related to Rate Base Investments		11,000
Public Utilities, Requested Increase Related to Generation and Transmission Expenses	[1]	6,800
Public Utilities, Requested Increase Related to Fuel and Purchased Power Expenses		11,000
Public Utilities, Total Requested Rate Increase Excluding Refunds		28,800
Public Utilities, Requested Decrease Related to Fuel Refunds		(9,500)
Public Utilities, Requested Decrease Related to Settlement Refund		$ (1,900)
Subsequent Event [Member] | NSP-Wisconsin | PSCW Proceeding - Wisconsin 2017 Electric and Gas Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Return on Equity, Percentage		10.00%
Public Utilities, Percentage of Excess Earnings to be Refunded due to Earnings Cap		100.00%
Subsequent Event [Member] | NSP-Wisconsin | PSCW Proceeding - Wisconsin 2017 Electric and Gas Rate Case - Gas Rates 2017 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 4,800
Public Utilities, Requested Rate Increase (Decrease), Percentage		3.90%
Public Utilities, Increase Related Environmental Remediation Expenses		$ 4,800

[1] Includes Interchange Agreement billings. The Interchange Agreement is a Federal Energy Regulatory Commission (FERC) tariff under which NSP-Wisconsin and its affiliate, NSP-Minnesota, own and operate a single integrated electric generation and transmission system and both companies pay a pro-rata share of system capital and operating costs. For financial reporting purposes, these expenses are included in operating and maintenance (O&M) expenses.