Rate Matters, NSP-Minnesota (Details) $ in Thousands				1 Months Ended							3 Months Ended	12 Months Ended
		Sep. 28, 2016	Jun. 30, 2016	Jan. 31, 2017 USD ($) project	Aug. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Feb. 28, 2015	Nov. 30, 2013	Mar. 31, 2015 USD ($)	Dec. 31, 2016 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												$ 0	$ 129,463	$ 0
NSP-Minnesota | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case
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%
NSP-Minnesota | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2016
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 194,600
Public Utilities, Requested Rate Increase (Decrease), Percentage							6.40%
Public Utilities, Requested Rate Base, Amount							$ 7,800,000
NSP-Minnesota | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2017
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 52,100
Public Utilities, Requested Rate Increase (Decrease), Percentage							1.70%
Public Utilities, Requested Interim Rate Increase (Decrease), Amount							$ 44,900
Public Utilities, Requested Rate Base, Amount							7,700,000
NSP-Minnesota | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2018
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 50,400
Public Utilities, Requested Rate Increase (Decrease), Percentage							1.70%
Public Utilities, Requested Rate Base, Amount							$ 7,700,000
NSP-Minnesota | MPUC Proceeding - Monticello 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
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 MISO 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
Rate Matters [Abstract]
Public Utilities, Approved Interim Rate Increase (Decrease), Amount						$ 163,700
NSP-Minnesota | Minnesota Public Utilities Commission | MPUC Proceeding - Monticello 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
NSP-Minnesota | Minnesota Public Utilities Commission | Electric Conservation Improvement Program (CIP) Rider 2016
Rate Matters [Abstract]
Public Utilities, Estimated average annual incentives												30,600
Public Utilities, CIP expenses recovered through rate rider												45,100
Public Utilities, CIP expenses recovered through base rates												90,200
NSP-Minnesota | Minnesota Public Utilities Commission | Natural Gas Conservation Improvement Program (CIP) Rider 2016
Rate Matters [Abstract]
Public Utilities, Estimated average annual incentives												3,600
Public Utilities, CIP expenses recovered through rate rider												15,400
Public Utilities, CIP expenses recovered through base rates												3,800
NSP-Minnesota | Minnesota Public Utilities Commission | Electric Conservation Improvement Program (CIP) Rider 2017 through 2019
Rate Matters [Abstract]
Public Utilities, Estimated average annual incentives												12,500
NSP-Minnesota | Minnesota Public Utilities Commission | Natural Gas Conservation Improvement Program (CIP) Rider 2017 through 2019
Rate Matters [Abstract]
Public Utilities, Estimated average annual incentives												1,800
NSP-Minnesota | Minnesota Public Utilities Commission | Electric Conservation Improvement Program (CIP) Rider 2015
Rate Matters [Abstract]
Public Utilities, CIP expenses recovered through rate rider												43,300
NSP-Minnesota | Minnesota Public Utilities Commission | Natural Gas Conservation Improvement Program (CIP) Rider 2015
Rate Matters [Abstract]
Public Utilities, CIP expenses recovered through rate rider												5,800
NSP-Minnesota | Minnesota Public Utilities Commission | Gas Utility Infrastructure Cost Rider 2017
Rate Matters [Abstract]
Public Utilities, Requested Rider Revenue, Amount												$ 22,100
NSP-Minnesota | Settlement Group | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Number Of Years Rate Case Is Applicable For					4 years
Public Utilities, Requested Return on Equity, Percentage					9.20%
Public Utilities, Requested Equity Capital Structure, Percentage					52.50%
Public Utilities, Cap on Annual True-Up for Decoupled Classes, Percentage					3.00%
Public Utilities, Cap on Annual True-Up, Percentage					3.00%
Public Utilities, Length of Stay-out Provision, In Years					4 years
Public Utilities, Rate Increase Under the Settlement	[1]				$ 184,970
Public Utilities, Increase (Decrease) Related to Sales True-up					59,750
Public Utilities, Rate Impact	[2]				244,720
NSP-Minnesota | Settlement Group | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2016
Rate Matters [Abstract]
Public Utilities, Rate Increase Under the Settlement	[1]				74,990
Public Utilities, Increase (Decrease) Related to Sales True-up					59,950
Public Utilities, Rate Impact	[2]				$ 134,940
Public Utilities, Rate Impact, Percentage					4.62%
NSP-Minnesota | Settlement Group | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2017
Rate Matters [Abstract]
Public Utilities, Rate Increase Under the Settlement	[1]				$ 59,860
Public Utilities, Increase (Decrease) Related to Sales True-up					0
Public Utilities, Rate Impact	[2]				$ 59,860
Public Utilities, Rate Impact, Percentage					2.05%
NSP-Minnesota | Settlement Group | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2018
Rate Matters [Abstract]
Public Utilities, Rate Increase Under the Settlement	[1]				$ 0
Public Utilities, Increase (Decrease) Related to Sales True-up					0
Public Utilities, Rate Impact	[2]				$ 0
Public Utilities, Rate Impact, Percentage					0.00%
NSP-Minnesota | Settlement Group | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2019
Rate Matters [Abstract]
Public Utilities, Rate Increase Under the Settlement	[1]				$ 50,120
Public Utilities, Increase (Decrease) Related to Sales True-up					(200)
Public Utilities, Rate Impact	[2]				$ 49,920
Public Utilities, Rate Impact, Percentage					1.71%
NSP-Minnesota | Administrative Law Judge | FERC Proceeding, MISO ROE Complaint
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties			9.70%
NSP-Minnesota | MPUC, NDPSC, SDPUC, and DOC | FERC Proceeding, MISO ROE Complaint
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties												8.81%
NSP-Minnesota | MISO TOs | FERC Proceeding, MISO ROE Complaint
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties												10.92%
NSP-Minnesota | FERC Staff | FERC Proceeding, MISO ROE Complaint
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties												8.78%
NSP-Minnesota | Federal Energy Regulatory Commission (FERC) | FERC Proceeding, MISO ROE Complaint
Rate Matters [Abstract]
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
Minimum | NSP-Minnesota | MPUC Proceeding - Monticello Project Prudency Investigation
Rate Matters [Abstract]
Public Utilities, Facility Generating Capacity | MW													600
Maximum | NSP-Minnesota | MPUC Proceeding - Monticello Project Prudency Investigation
Rate Matters [Abstract]
Public Utilities, Facility Generating Capacity | MW													671
Subsequent Event | NSP-Minnesota | MPUC Proceeding - Transmission Cost Recovery Rider Filing 2016
Rate Matters [Abstract]
Public Utilities, Additional Transmission Investment, Number Of Projects | project				3
Public Utilities, Additional Transmission Investment, Number Of Additional Projects | project				2
Subsequent Event | NSP-Minnesota | Minnesota Public Utilities Commission | MPUC Proceeding - Transmission Cost Recovery Rider Filing 2016
Rate Matters [Abstract]
Public Utilities, Approved Rider Revenue, Amount				$ 78,400

[1] The settlement revenues are based on the DOC’s sales forecast.

[2] The total rate impact reflects an increase of 4.62 percent in 2016; 2.05 percent in 2017; 0 percent in 2018 and 1.71 percent in 2019.