Rate Matters, NSP-Minnesota (Details) (NSP-Minnesota, 2012 Electric Rate Case [Member], USD $) In Millions, unless otherwise specified	1 Months Ended			3 Months Ended		1 Months Ended	0 Months Ended	1 Months Ended			0 Months Ended
	Mar. 31, 2013 Minnesota Public Utilities Commission (MPUC) [Member]	Jan. 31, 2013 Minnesota Public Utilities Commission (MPUC) [Member]	Nov. 30, 2012 Minnesota Public Utilities Commission (MPUC) [Member]	Jun. 30, 2013 Minnesota Public Utilities Commission (MPUC) [Member]	Mar. 31, 2013 Minnesota Public Utilities Commission (MPUC) [Member]	Apr. 30, 2013 Minnesota Department of Commerce [Member]	Jul. 03, 2013 Administrative Law Judge [Member]	Jun. 30, 2013 North Dakota Public Service Commission (NDPSC) [Member]	Jan. 31, 2013 North Dakota Public Service Commission (NDPSC) [Member]	Dec. 31, 2012 North Dakota Public Service Commission (NDPSC) [Member]	Jul. 17, 2013 NDPSC Advocacy Staff [Member]
Rate Matters [Abstract]
Entity's requested rate increase (decrease)			$ 285.0							$ 16.9
Percentage increase in entity's requested rate increase (in hundredths)			10.70%							9.25%
Entity's requested return on equity (in hundredths)	10.60%		10.60%							10.60%
Entity's rate base in requested rate increase (decrease)	6,300.0		6,300.0							377.6
Entity's requested equity ratio percent (in hundredths)	52.56%		52.56%							52.56%
Entity's interim rate increase (decrease)		251.0							14.7
Entity's revised requested rate increase (decrease)	219.7			209.0				16.0
Percentage increase in entity's revised requested rate increase (in hundredths)	8.23%
Public Utilities, Percent of limitation of base pay for calculating annual incentive plan	15.00%
Public Utilities, term (in years) used in calculation of annual incentive plan	four-year
Public Utilities, term (in years) used in amortization for recovery of environmental remediation costs	15 years
Public Utilities, term (in months) used in actual sales in updated forecast	6 months
Public Utilities, Rate Increase (decrease) recommended by third parties						89.6	127.0				(2.1)
Return on equity recommended by third parties (in hundredths)						9.83%	9.83%				9.00%
Rate base recommended by third parties						6,100	6,233
Equity ratio percent recommended by third parties (in hundredths)						52.56%	52.56%
Public Utilities, Rate increase (decrease) subsequently recommended by third parties						98.6
Public Utilities, Requested Rate Increase Including Deferral Mechanisms				259
Public Utilities, Deferral Mechanisms for Rate Mitigation				50
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to deferrals							51
Public Utilities, Deferral Mechanism for Rate Mitigation requested by third parties						5	34
Public Utilities, Adjustment requested by public utility related to lower return on equity	0
Public Utilities, Adjustment requested by third parties related to lower return on equity						(43.0)	(43.0)				(5.2)
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to removal of avoided costs of electric generation unit	(35)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to removal of avoided costs of electric generation unit						(40)	(38)
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to reduced recovery for nuclear plants	(11)
Reduced recovery for the nuclear plants						(9)	(14)
Public Utilities, Adjustment to requested by public utility to rate increase (decrease) related to elimination of certain incentive compensation	(3)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to incentive compensation						(20.0)	(4.0)				(0.8)
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to an increase to sales forecast	(1)
Increase to the sales forecast						(26)	(26)
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to reduced recovery for pension benefits	(10)
Reduced recovery of pension						(25)	(13)
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to reduced recovery for employee benefits	(4)
Employee benefits						(6)	(6)
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to Black Dog remediation costs	(5)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to Black Dog remediation costs						(5)	(5)
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to NSP-Wisconsin wholesale allocation				(7)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to NSP-Wisconsin wholesale allocation						(7)	(7)
Public Utilities, Adjustment requested by public utility to rate increase (decrease) related to other costs				0
Other, net						(5.0)	(2.0)				0.9
Public Utilities, Recommended rate increase - estimated impact on 2013 pre-tax income						104	161
Public Utility, Liability representing the current best estimate of a refund obligation				47	16
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to use of a one month coincident peak demand allocator for certain rate base and operation exps											(20.0)
Public Utilities, Number of months used for coincident peak demand allocator for certain rate base and operation expenses											1 month
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to various O&M expenses											(0.7)
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to calculation of federal income taxes											6.3
Public Utilities, Adjustment to requested rate increase (decrease) requested by third parties related to modified cost of capital and increased capital structure											$ 1.4
Public Utilities, Cost of Capital and Capital Structure requested by third parties											53.42%