Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled	$ 0.0	$ 0.0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	14.4	11.4
Asset Retirement Obligation, Liabilities Incurred	0.0	1.0
Accretion	0.5	0.3
Cash Flow Revisions	3.0	1.7
Ending balance	17.9	14.4
Electric Plant Steam Production Asbestos
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled	0.0	0.0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	3.7	2.7
Asset Retirement Obligation, Liabilities Incurred	0.0	1.0
Accretion	0.1	0.0
Cash Flow Revisions	0.0	0.0
Ending balance	3.8	3.7
Electric Plant Electric Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	0.0
Accretion	0.0
Cash Flow Revisions	4.6
Ending balance	4.6	0.0
Electric Plant Other
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled	0.0	0.0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	0.4	0.4
Asset Retirement Obligation, Liabilities Incurred	0.0	0.0
Accretion	0.0	0.0
Cash Flow Revisions	0.0	0.0
Ending balance	0.4	0.4
Natural Gas Plant Gas Distribution
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled	0.0	0.0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	10.3	8.3
Asset Retirement Obligation, Liabilities Incurred	0.0	0.0
Accretion	0.4	0.3
Cash Flow Revisions	(1.6)	1.7
Ending balance	9.1	10.3
Removal Costs
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory Liabilities	$ 158.0	$ 146.0

-- None. No documentation exists for this element. --

No definition available.

Name:	nspw_AssetRetirementObligationsLineItems
Namespace Prefix:	nspw_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

+ Details

Name:	us-gaap_AssetRetirementObligationAccretionExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of asset retirement obligations incurred during the period.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationLiabilitiesIncurred
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationLiabilitiesSettled
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 35
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

+ Details

Name:	us-gaap_AssetRetirementObligationRevisionOfEstimate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_AssetRetirementObligationRollForwardAnalysisRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

+ Details

Name:	us-gaap_RegulatoryLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	nspw_AssetRetirementObligationsByClassOfPropertyAxis=nspw_ElectricPlantSteamProductionAsbestosMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	nspw_AssetRetirementObligationsByClassOfPropertyAxis=us-gaap_ElectricDistributionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	nspw_AssetRetirementObligationsByClassOfPropertyAxis=nspw_ElectricPlantOtherSourcesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	nspw_AssetRetirementObligationsByClassOfPropertyAxis=nspw_NaturalGasPlantGasTransmissionAndDistributionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryLiabilityAxis=us-gaap_RemovalCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: