Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Other MGP Sites [Member]
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 4	$ 4
Number of identified MGP sites under current investigation and/or remediation in addition to those separately disclosed	11
NSP-Wisconsin \| Ashland MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 28	30
Current Cost Estimate for Site Remediation	184
Estimated amount spent on cleanup	$ 156
Approved amortization period for recovery of remediation costs in natural gas rates (in years)	10
Carrying cost percentage to be applied to unamortized regulatory asset	3.00%
NSP-Minnesota \| Fargo MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 6	16
Current Cost Estimate for Site Remediation	25
Estimated amount spent on cleanup	$ 19
Percentage of Response Costs Allocable to the Minnesota Jurisdiction	88.00%
PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2017 \| NSP-Wisconsin \| Ashland MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Requested annual recovery collected through base rates		$ 12
PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2018 [Member] \| NSP-Wisconsin \| Ashland MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Requested annual recovery collected through base rates	$ 18

Undiscounted amount of the accrual for environmental loss contingencies.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 5.Y.Q1)
-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

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The amortization period for recovery of remediation costs in natural gas rates, approved by the Public Service Commission of Wisconsin (PSCW).

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The number of sites where former manufactured gas plant (MGP) activities have or may have resulted in actual site contamination and are under current investigation and/or remediation.

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Percentage of Response Costs Allocable to the Minnesota Jurisdiction

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Public Utilities, Requested annual recovery collected through base rates

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