Commitments and Contingencies, Nuclear Insurance (Details) - NSP-Minnesota - Nuclear Insurance $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Plant Reactor	Jan. 01, 2017 USD ($)
Nuclear Insurance [Abstract]
Nuclear insurance coverage secured for the Company's public liability exposure	$ 375.0	$ 450.0
Nuclear insurance coverage exposure funded by the Secondary Financial Protection Program	13,000.0
Maximum assessments per reactor per accident	$ 127.3
Number of owned and licensed reactors | Reactor	3
Maximum funding requirement per reactor for any one year	$ 19.0
Term for maximum installment payment assessment per reactor (in years)	1 year
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	$ 19.8
Maximum assessment for property damage insurance NSP-Minnesota is subject to each calendar year	43.0
Maximum
Nuclear Insurance [Abstract]
Loss Contingency, Estimate of Possible Loss	$ 13,400.0