Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) Plant
Clean Water Act Effluent Limitations Guidelines [Domain]
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation	$ 12
PSCo \| Federal Coal Ash Regulation [Domain]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of sites where statistically significant increases over established groundwater standards exist	2
NSP-Minnesota \| Federal Coal Ash Regulation [Domain]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of sites where SSI's have been identified	1
NSP-Minnesota \| Federal Clean Water Act Section 316(b)
Environmental Requirements [Abstract]
Minimum Number of Plants Which Could Be Required to Make Improvements to Reduce Entrainment \| Plant	6
NSP-Wisconsin \| Federal Clean Water Act Section 316(b)
Environmental Requirements [Abstract]
Minimum Number of Plants Which Could Be Required to Make Improvements to Reduce Entrainment \| Plant	2
Capital Commitments \| Federal Clean Water Act Section 316(b)
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation	$ 40
Liability for Estimated Cost to Comply With Entrainment Regulation	$ 200
Harrington Units 1 and 2 \| SPS \| Implementation of the National Ambient Air Quality Standard for Sulfur Dioxide
Environmental Requirements [Abstract]
Number of Years Unclassifiable Areas Will Be Monitored	3 years
Tolk Units 1 and 2 \| Capital Commitments \| SPS \| Regional Haze Rules
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation	$ 600

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Estimated undiscounted amount of the accrual required to comply with required regulation.

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Estimated undiscounted amount of the accrual required to comply with required entrainment regulation under the federal Clean Water Act Section 316(b) rule.

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- Definition

Minimum number of plants which could be required by state regulators to make improvements to reduce entrainment

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Number of impoundments where closure plans will be expedited

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Number of sites where statistically significant increases over established groundwater standards exist

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Number of years unclassifiable areas will be monitored

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