Commitments and Contingencies, Joint Operating System (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Plant Reactor Counterparty
Joint Operating System [Abstract]
Number of companies covered by FERC approved Interchange Agreement | Counterparty	2
NSP-Minnesota | Nuclear Insurance
Joint Operating System [Abstract]
Nuclear insurance coverage secured for the Company's public liability exposure	$ 450
Nuclear insurance coverage exposure funded by the Secondary Financial Protection Program	13,000
Maximum assessments per reactor per accident	$ 127
Number of owned and licensed reactors | Reactor	3
Maximum funding requirement per reactor for any one year	$ 19
Insurance coverage limits for NSP-Minnesota's nuclear plant sites	$ 2,300
Number of nuclear plant sites operated by NSP-Minnesota | Plant	2
Maximum assessments for business interruption insurance each calendar year	$ 19
Maximum assessment for property damage insurance NSP-Minnesota is subject to each calendar year	41
Maximum | NSP-Minnesota | Nuclear Insurance
Joint Operating System [Abstract]
Maximum possible loss contingency	$ 13,400