Commitments and Contingent Liabilities, Asset Retirement Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 969,310,000		$ 881,479,000
Liabilities recognized	0		29,474,000
Liabilities settled	(514,000)		(2,330,000)
Accretion	66,903,000		58,223,000
Revisions to prior estimates	616,094,000		2,464,000
Ending balance	1,651,793,000		969,310,000
Assets held in external decommissioning trust	1,300,000,000		1,400,000,000
Electric Plant - Steam Production Asbestos [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	93,629,000		95,093,000
Liabilities recognized	0		3,771,000
Liabilities settled	(514,000)		(2,330,000)
Accretion	5,958,000		6,037,000
Revisions to prior estimates	(44,731,000)		(8,942,000)
Ending balance	54,342,000		93,629,000
Electric Plant - Steam Production Ash Containment [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	19,688,000		17,552,000
Liabilities recognized	0		32,000
Liabilities settled	0		0
Accretion	919,000		903,000
Revisions to prior estimates	20,551,000		1,201,000
Ending balance	41,158,000		19,688,000
Electric Plant - Steam Production Radiation Sources [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	166,000		176,000
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	12,000		12,000
Revisions to prior estimates	(39,000)		(22,000)
Ending balance	139,000		166,000
Electric Plant - Nuclear Production Decommissioning [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	809,474,000		758,923,000
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	57,641,000		50,551,000
Revisions to prior estimates	615,626,000	[1]	0
Ending balance	1,482,741,000		809,474,000
Electric Plant - Wind Production [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	38,553,000		7,751,000
Liabilities recognized	0		25,671,000
Liabilities settled	0		0
Accretion	1,962,000		592,000
Revisions to prior estimates	0		4,539,000
Ending balance	40,515,000		38,553,000
Electric Plant - Electric Transmission and Distribution [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	5,727,000		27,000
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	290,000		12,000
Revisions to prior estimates	24,687,000		5,688,000
Ending balance	30,704,000		5,727,000
Natural Gas Plant - Gas Transmission and Distribution [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	996,000		936,000
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	63,000		60,000
Revisions to prior estimates	0		0
Ending balance	1,059,000		996,000
Common and Other Property - Common General Plant Asbestos [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,077,000		1,021,000
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	58,000		56,000
Revisions to prior estimates	0		0
Ending balance	$ 1,135,000		$ 1,077,000

[1] The increase is primarily due to the completion of NSP-Minnesota's triennial nuclear decommissioning study, which reflects an increase in the estimated cost of retirement, increase in the escalation rates for each nuclear unit and a decrease in the discount rate used to calculate the net present value of the future cash flows.