Rate Matters, NSP-Minnesota (Details) $ in Thousands		1 Months Ended										3 Months Ended	12 Months Ended
	Jan. 06, 2015	Jan. 31, 2016	Dec. 31, 2015 USD ($)	Nov. 30, 2015 USD ($) project	Oct. 31, 2015 USD ($) project	Jul. 31, 2015 USD ($)	May. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Feb. 28, 2015	Dec. 31, 2013 USD ($)	Nov. 30, 2013 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2008 USD ($)
Rate Matters [Abstract]
Loss on Monticello life cycle management/extended power uprate project													$ 129,463	$ 0	$ 0
NSP-Minnesota | MPUC Proceeding - Minnesota Electric Rate Case 2014
Public Utilities, General Disclosures [Line Items]
Public Utilities, Number Of Years Rate Case Is Applicable For											2 years
Rate Matters [Abstract]
Public Utilities, Requested Return on Equity, Percentage											10.25%
Public Utilities, Requested Equity Capital Structure, Percentage											52.50%
NSP-Minnesota | MPUC Proceeding - Minnesota Electric Rate Case 2014, Rates 2014
Rate Matters [Abstract]
Public Utilities, Requested Rate Base, Amount											$ 6,670,000
Public Utilities, Requested Rate Increase (Decrease), Amount											$ 193,000
Public Utilities, Requested Rate Increase (Decrease), Percentage											6.90%
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount														115,300
NSP-Minnesota | MPUC Proceeding - Minnesota Electric Rate Case 2014, Rates 2015
Rate Matters [Abstract]
Public Utilities, Requested Rate Base, Amount											$ 412,000
Public Utilities, Requested Rate Increase (Decrease), Amount											$ 98,000
Public Utilities, Requested Rate Increase (Decrease), Percentage											3.50%
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount														106,000
Public Utilities, Amount Of Public Utility's Amended Unadjusted Cumulative Multi-year Rate Increase														$ 221,300
NSP-Minnesota | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Number Of Years Rate Case Is Applicable For				3 years
Rate Matters [Abstract]
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 Base, Amount				7,800,000
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 194,600
Public Utilities, Requested Rate Increase (Decrease), Percentage				6.40%
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 Base, Amount				7,700,000
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 52,100
Public Utilities, Requested Rate Increase (Decrease), Percentage				1.70%
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 Base, Amount				7,700,000
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 50,400
Public Utilities, Requested Rate Increase (Decrease), Percentage				1.70%
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 - 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 | 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 | project				2	2
NSP-Minnesota | Electric Conservation Improvement Program (CIP) Rider 2016 [Member]
Rate Matters [Abstract]
Public Utilities, CIP expenses recovered through rate rider						$ 21,500
NSP-Minnesota | Natural Gas Conservation Improvement Program (CIP) Rider 2016 [Member]
Rate Matters [Abstract]
Public Utilities, CIP expenses recovered through rate rider						9,200
NSP-Minnesota | Electric Conservation Improvement Program (CIP) Rider 2015 [Member]
Rate Matters [Abstract]
Public Utilities, CIP expenses recovered through base rates						86,900
NSP-Minnesota | Natural Gas Conservation Improvement Program (CIP) Rider 2015 [Member]
Rate Matters [Abstract]
Public Utilities, CIP expenses recovered through base rates						3,700
NSP-Minnesota | Gas Utility Infrastructure Cost Rider 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rider Revenue, Amount					$ 15,500
Public Utilities, Over-recovered Rider Costs					1,900
Public Utility, Deferred sewer separation costs					$ 4,500
Public Utilities, Length of Amortization Period, In Years					5 years
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%
Public Utilities, ROE applicable to transmission formula rates in the MISO region, recommended by third parties									8.67%		9.15%
Public Utilities, maximum equity capital structure percentage allowed per the complaint											50.00%
Public Utilities, ROE applicable to transmission formula rates in the MISO region, lower bound, percentage													8.67%
Public Utilities, ROE applicable to transmission formula rates in the MISO region, upper bound, percentage													9.54%
NSP-Minnesota | Minnesota Public Utilities Commission | MPUC Proceeding - Minnesota Electric Rate Case 2014
Rate Matters [Abstract]
Public Utilities, Approved Rate Increase (Decrease), Amount						$ 149,400	$ 166,100
Public Utilities, Approved Rate Increase (Decrease), Percentage						5.30%	5.90%
Public Utilities; Approved Decrease Related To Disallowance Of Project Costs							$ 18,000
Public Utilities, Number Of Years Of Decoupling Pilot							3 years
Public Utilities, Percent Cap On Revenue For Decoupling Pilot							3.00%
Public Utilities, Approved Decrease Related To Used-and-useful Date Of Project						$ (13,800)
Estimated Revenue Impact						145,800
NSP-Minnesota | Minnesota Public Utilities Commission | MPUC Proceeding - Minnesota Electric Rate Case 2014, Rates 2014
Rate Matters [Abstract]
Public Utilities, Interim Rate Increase (Decrease), Amount										$ 127,000
Public Utilities, Approved Rate Increase (Decrease), Amount							$ 58,900
Public Utilities, Approved Decrease Related To Property Taxes						(3,100)
NSP-Minnesota | Minnesota Public Utilities Commission | MPUC Proceeding - Minnesota Electric Rate Case 2014, Rates 2015
Rate Matters [Abstract]
Public Utilities, Approved Rate Increase (Decrease), Amount							$ 125,200
Public Utilities, Approved Increase Related To Other, Net						200
Public Utilities, Decrease Related To Interim Rates Effective Date						(3,600)
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 | Minnesota Public Utilities Commission | Electric Conservation Improvement Program (CIP) Rider 2013 through 2016
Rate Matters [Abstract]
Public Utilities, Estimated average annual incentives													$ 30,600
NSP-Minnesota | Minnesota Public Utilities Commission | Natural Gas Conservation Improvement Program (CIP) Rider 2013 through 2016
Rate Matters [Abstract]
Public Utilities, Estimated average annual incentives													$ 3,600
NSP-Minnesota | Minnesota Public Utilities Commission | Electric Conservation Improvement Program (CIP) Rider 2014 [Member]
Rate Matters [Abstract]
Public Utilities, CIP expenses recovered through rate rider						40,100
NSP-Minnesota | Minnesota Public Utilities Commission | Natural Gas Conservation Improvement Program (CIP) Rider 2014 [Member]
Rate Matters [Abstract]
Public Utilities, CIP expenses recovered through rate rider						$ 5,800
NSP-Minnesota | South Dakota Public Utilities Commission [Member] | SDPUC Proceeding - Infrastructure Rider Filing 2016 [Member]
Rate Matters [Abstract]
Public Utilities, Approved Rider Revenue, Amount			$ 10,200
NSP-Minnesota | Administrative Law Judge | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE applicable to transmission formula rates in the MISO region, recommended by third parties			10.32%
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 | FERC Staff [Member] | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE applicable to transmission formula rates in the MISO region, recommended by third parties													8.68%
NSP-Minnesota | MISO TOs [Member] | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE applicable to transmission formula rates in the MISO region, lower bound, percentage													10.80%
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
Subsequent Event | NSP-Minnesota | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE applicable to transmission formula rates in the MISO region, lower bound, percentage		8.72%
Public Utilities, ROE applicable to transmission formula rates in the MISO region, upper bound, percentage		9.32%
Subsequent Event | NSP-Minnesota | FERC Staff [Member] | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE applicable to transmission formula rates in the MISO region, recommended by third parties		8.78%
Subsequent Event | NSP-Minnesota | MISO TOs [Member] | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE applicable to transmission formula rates in the MISO region, recommended by third parties		10.96%