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Commitments and Contingencies, Joint Operating System (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Plant
Counterparty
Reactor
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 $ 450.0
Nuclear insurance coverage exposure funded by the Secondary Financial Protection Program 13,600.0
Maximum assessments per reactor per accident $ 137.6
Number of owned and licensed reactors | Reactor 3
Maximum funding requirement per reactor for any one year $ 20.5
Insurance coverage limits for NSP-Minnesota's nuclear plant sites $ 2,300.0
Number of nuclear plant sites operated by NSP-Minnesota | Plant 2
Maximum assessments for business interruption insurance each calendar year $ 18.0
Maximum assessment for property damage insurance NSP-Minnesota is subject to each calendar year 39.0
NSP-Minnesota | Maximum | Nuclear Insurance  
Joint Operating System [Abstract]  
Maximum possible loss contingency $ 14,100.0