Commitments and Contingencies, Joint Operating System (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 Counterparty
Joint Operating System [Abstract]
Number of companies covered by FERC approved Interchange Agreement	2
NSP-Minnesota | Nuclear Insurance
Joint Operating System [Abstract]
Maximum possible loss contingency	13,600,000,000
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	13,200,000,000
Maximum assessments per reactor per accident	127,300,000
Number of owned and licensed reactors	3
Maximum funding requirement per reactor for any one year	19,000,000
Term for maximum installment payment assessment per reactor (in years)	1 year
Insurance coverage limits for NSP-Minnesota's nuclear plant sites	2,300,000,000
Number of nuclear plant sites operated by NSP-Minnesota	2
Maximum assessments for business interruption insurance each calendar year	17,900,000
Maximum assessment for property damage insurance NSP-Minnesota is subject to each calendar year	43,600,000