Rate Matters, NSP-Minnesota (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended	1 Months Ended			3 Months Ended	1 Months Ended	3 Months Ended	1 Months Ended
	Oct. 31, 2013 Nuclear Project Prudency Investigation [Member] [Member]	Mar. 31, 2014 Nuclear Project Prudency Investigation [Member] [Member]	Dec. 31, 2013 NSP-Minnesota Minnesota Public Utilities Commission [Member] 2014 Electric Rate Case	Nov. 30, 2013 NSP-Minnesota Minnesota Public Utilities Commission [Member] 2014 Electric Rate Case	Nov. 30, 2013 NSP-Minnesota Minnesota Public Utilities Commission [Member] Electric Rate Case 2014, Rates 2014 [Member]	Mar. 31, 2014 NSP-Minnesota Minnesota Public Utilities Commission [Member] Electric Rate Case 2014, Rates 2014 [Member]	Nov. 30, 2013 NSP-Minnesota Minnesota Public Utilities Commission [Member] Electric Rate Case 2014, Rates 2015 [Member]	Mar. 31, 2014 NSP-Minnesota Minnesota Public Utilities Commission [Member] Electric Rate Case 2014, Rates 2015 [Member]	Feb. 28, 2014 NSP-Minnesota North Dakota Public Service Commission [Member] Electric Rate Case 2013 [Member]	Jan. 31, 2013 NSP-Minnesota North Dakota Public Service Commission [Member] Electric Rate Case 2013 [Member]	Dec. 31, 2012 NSP-Minnesota North Dakota Public Service Commission [Member] Electric Rate Case 2013 [Member]	Feb. 28, 2014 NSP-Minnesota North Dakota Public Service Commission [Member] Electric Rate Case 2013, Rates 2013	Feb. 28, 2014 NSP-Minnesota North Dakota Public Service Commission [Member] Electric Rate Case 2013, Rates 2014 [Member]	Feb. 28, 2014 NSP-Minnesota North Dakota Public Service Commission [Member] Electric Rate Case 2013, Rates 2015 [Member]	Feb. 28, 2014 NSP-Minnesota North Dakota Public Service Commission [Member] Electric Rate Case 2013, ROE 2013 [Member]	Feb. 28, 2014 NSP-Minnesota North Dakota Public Service Commission [Member] Electric Rate Case 2013, ROE 2014 [Member]	Feb. 28, 2014 NSP-Minnesota North Dakota Public Service Commission [Member] Electric Rate Case 2013, ROE 2015 [Member]	Feb. 28, 2014 NSP-Minnesota North Dakota Public Service Commission [Member] Electric Rate Case 2013, ROE 2016 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revenue deficiency based on a forecast test year					$ 274		$ 81
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					193.0	127.0	98.0	164.0			16.9
Public Utilities, Requested Rate Increase (Decrease), Percentage					6.90%	4.60%	3.50%	5.60%	4.90%		9.25%
Public Utilities, Impact on Customer Bill, Increase (Decrease), Percentage					4.60%		5.60%
Public Utilities, Requested Return on Equity, Percentage				10.25%							10.60%
Public Utilities, Requested Rate Base, Amount					6,670.0		412.0				377.6
Public Utilities, Requested Equity Capital Structure, Percentage				52.50%							52.56%
Public Utilities, Interim Rate Increase (Decrease), Amount			127.0							14.7
Public Utilities, Cost of project allowed for recovery		320
Costs for nuclear project, Amount		665
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to depreciation reserve					(81)		53
Number of factors attributable to project cost increases	3
Number of years for the application process	5
Public Utilities, Recommended rate increase (decrease) impact on pre-tax income					224		154
Public Utilities, Approved Rate Increase (Decrease), Amount												7.4	9.4	10.1
Public Utilities, Approved Return on Equity, Percentage															9.75%	10.00%	10.00%	10.25%
Proposed duration of rate plan, years									4 years
Public utilities, Adjustment to requested rate increase (decrease) related to DOE settlement proceeds, amount					0		(36)
Public Utilities, Adjustment to requested interim rate increase (decrease), amount					(66)		66
Public Utilities, Impact on Customer Bill, Increase (Decrease), Amount					127		164
Amortization of Rate Deferral					16		0
Public Utilities, Adjustment to approved rate increase (decrease) related to depreciation expense					$ 81		$ (46)