Nuclear Insurance (Details) - NSP Minnesota - Nuclear Insurance $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Plant Reactor
Nuclear Insurance [Abstract]
Nuclear insurance coverage secured for the Company's public liability exposure	$ 450
Nuclear insurance coverage exposure funded by the Secondary Financial Protection Program	13,500
Maximum assessments per reactor per accident	$ 138
Number of owned and licensed reactors | Reactor	3
Maximum funding requirement per reactor for any one year	$ 21
Number of nuclear plant sites operated by NSP-Minnesota | Plant	2
Maximum assessments for business interruption insurance each calendar year	$ 12
Maximum assessment for property damage insurance NSP-Minnesota is subject to each calendar year	35
Maximum
Nuclear Insurance [Abstract]
Loss Contingency, Estimate of Possible Loss	13,900
Insurance coverage limits for NSP-Minnesota's nuclear plant sites	2,700
Interruption Insurance Coverage Limits	$ 350