Commitments and Contingent Liabilities, Nuclear Insurance (Details) (Nuclear Insurance [Member], USD $)	12 Months Ended
	Dec. 31, 2011
Nuclear Insurance [Member]
Nuclear Insurance [Abstract]
Maximum possible loss contingency	$ 12,600,000,000
Nuclear insurance coverage secured for the company's public liability exposure	375,000,000
Nuclear insurance coverage exposure funded by the Secondary Financial Protection Program	12,200,000,000
Maximum assessments per reactor per accident	117,500,000
Number of owned and licensed reactors	3
Maximum funding requirement per reactor for any one year	17,500,000
Term for maximum installment payment assessment per reactor (in years)	1Y
Insurance coverage limits for NSP-Minnesota's nuclear plant sites	2,250,000,000
Number of nuclear plant sites operated by NSP-Minnesota	2
Maximum assessments for business interruption insurance each calendar year	15,700,000
Maximum assessment for property damage insurance NSP-Minnesota is subject to each calendar year	$ 33,600,000