Rate Matters (Details) $ in Thousands		1 Months Ended							3 Months Ended		12 Months Ended
	Jan. 06, 2015	Dec. 31, 2015	Nov. 30, 2015 USD ($) project	Oct. 31, 2015 USD ($)	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 ($)
Rate Matters [Abstract]
Loss on Monticello life cycle management/extended power uprate project									$ 0	$ 124,226
NSP-Minnesota | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case [Member]
Rate Matters [Abstract]
Public Utilities, Number Of Years Rate Case Is Applicable For			3 years
Public Utilities, Requested Return on Equity, Percentage			10.00%
Public Utilities, Requested Equity Capital Structure, Percentage			52.50%
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 297,100
Public Utilities, Number Of Years Rate Case is Applicable For, Alternative Plan			5 years
Public Utilities, Number Of Additional Years Under Alternative Rate Plan			2 years
Public Utilities, Increase Related to Excess Depreciation Reserve			$ 77,000
Public Utilities, Increase Related to Settlement			25,700
Public Utilities, Increase Related to Nuclear Projects			8,100
Public Utilities, Total Increase Related to 2014 Multi-Year Rate Case Items			110,800
Public utilities, Requested Increase Related to Capital Investment			186,100
Public Utilities, Increase Related To Property Taxes			43,000
Public Utilities, Decrease Related To Net Operating Loss Carryforwards			(37,300)
Public Utilities, Decrease related to other, net			(5,500)
Public Utilities, Total Increase Related To Additional Items			$ 186,300
NSP-Minnesota | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Percentage			6.40%
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 194,600
Public Utilities, Requested Rate Base, Amount			7,800,000
Public Utilities, Increase Related to Excess Depreciation Reserve			26,000
Public Utilities, Increase Related to Settlement			25,700
Public Utilities, Increase Related to Nuclear Projects			11,200
Public Utilities, Total Increase Related to 2014 Multi-Year Rate Case Items			62,900
Public utilities, Requested Increase Related to Capital Investment			128,700
Public Utilities, Increase Related To Property Taxes			30,200
Public Utilities, Decrease Related To Net Operating Loss Carryforwards			(6,300)
Public Utilities, Decrease related to other, net			(20,900)
Public Utilities, Total Increase Related To Additional Items			$ 131,700
NSP-Minnesota | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2017 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Percentage			1.70%
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 52,100
Public Utilities, Requested Rate Base, Amount			7,700,000
Public Utilities, Increase Related to Excess Depreciation Reserve			51,000
Public Utilities, Increase Related to Settlement			0
Public Utilities, Decrease Related to Nuclear Projects			(1,600)
Public Utilities, Total Increase Related to 2014 Multi-Year Rate Case Items			49,400
Public utilities, Requested Increase Related to Capital Investment			12,800
Public Utilities, Increase Related To Property Taxes			7,600
Public Utilities, Decrease Related To Net Operating Loss Carryforwards			(24,500)
Public Utilities, Increase Related To Other, Net			6,800
Public Utilities, Total Increase Related To Additional Items			$ 2,700
NSP-Minnesota | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2018 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Percentage			1.70%
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 50,400
Public Utilities, Requested Rate Base, Amount			7,700,000
Public Utilities, Increase Related to Excess Depreciation Reserve			0
Public Utilities, Increase Related to Settlement			0
Public Utilities, Decrease Related to Nuclear Projects			(1,500)
Public Utilities, Total Decrease Related to 2014 Multi-Year Rate Case Items			(1,500)
Public utilities, Requested Increase Related to Capital Investment			44,600
Public Utilities, Increase Related To Property Taxes			5,200
Public Utilities, Decrease Related To Net Operating Loss Carryforwards			(6,500)
Public Utilities, Increase Related To Other, Net			8,600
Public Utilities, Total Increase Related To Additional Items			51,900
NSP-Minnesota | MPUC Proceeding - Transmission Cost Recovery Rider Filing 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rider Revenue, Amount				$ 19,200
NSP-Minnesota | MPUC Proceeding - Transmission Cost Recovery Rider Filing 2016 - With Additional Transmission Investment [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rider Revenue, Amount			$ 78,300
Public Utilities, Additional Transmission Investment, Amount				$ 59,100
Public Utilities, Additional Transmission Investment, Number Of Projects			2	2
NSP-Minnesota | MPUC Proceeding - Nuclear Project Prudency Investigation
Rate Matters [Abstract]
Nuclear Project Expenditures, Amount												$ 665,000
Total Capitalized Nuclear Project Costs												$ 748,000
Initial Estimated Nuclear Project Expenditures													$ 320,000
Loss on Monticello life cycle management/extended power uprate project										$ 129,000
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
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 | Minnesota Public Utilities Commission | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Interim Rate Increase (Decrease), Amount			$ 163,700
NSP-Minnesota | Minnesota Public Utilities Commission | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2017 [Member]
Rate Matters [Abstract]
Public Utilities, Interim Rate Increase (Decrease), Amount			$ 44,900
NSP-Minnesota | Minnesota Public Utilities Commission | MPUC Proceeding - Nuclear Project Prudency Investigation
Rate Matters [Abstract]
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 | Federal Energy Regulatory Commission (FERC) | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
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 | Administrative Law Judge | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The Regional Transmission Operator's Region, Recommended By Third Parties		10.32%
NSP-Minnesota | FERC Staff [Member] | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The Regional Transmission Operator's Region, Recommended By Third Parties									8.78%
NSP-Minnesota | MISO TOs [Member] | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The Regional Transmission Operator's Region, Recommended By Third Parties									10.92%
NSP-Minnesota | MPUC, NDPSC, SDPUC, and DOC [Member] | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The Regional Transmission Operator's Region, Lower Bound, Percentage									8.81%
Public Utilities, ROE Applicable To Transmission Formula Rates In The Regional Transmission Operator's Region, Upper Bound, Percentage									8.82%
Minimum | NSP-Minnesota | MPUC Proceeding - Nuclear Project Prudency Investigation
Rate Matters [Abstract]
Public Utilities, Facility Generating Capacity, in MW | MW											600
Minimum | NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
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
Rate Matters [Abstract]
Public Utilities, Facility Generating Capacity, in MW | MW											671
Maximum | NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, Decrease In Transmission Revenue, Net Of Expense, Due To New ROE Methodology									$ 10,000