Benefit Plans and Other Postretirement Benefits, Projected Benefit Payments (Details) $ in Millions	Dec. 31, 2018 USD ($)
Pension Plan [Member]
Defined Benefit Plan, Gross Projected Benefit Payments [Abstract]
2019	$ 14.6
2020	10.9
2021	10.9
2022	10.8
2023	11.1
2024-2028	55.5
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Gross Projected Benefit Payments [Abstract]
2019	1.2
2020	1.1
2021	1.1
2022	1.0
2023	1.0
2024-2028	4.2
Expected Medicare Part D Subsidies [Abstract]
2019	0.0
2020	0.0
2021	0.0
2022	0.0
2023	0.0
2024-2028	0.0
Defined Benefit Plan, Net Projected Benefit Payments [Abstract]
2019	1.2
2020	1.1
2021	1.1
2022	1.0
2023	1.0
2024-2028	$ 4.2

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

No definition available.

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

- Definition

The amount of the benefits expected to be paid, net of the prescription drug subsidy receipts expected to be received in the five fiscal years thereafter (the aggregate net amount of benefits expected to be paid in years 6 through 10 after the date of the latest statement of financial position).

+ References

No definition available.