Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Dividend and Other Capital-Related Restrictions (Details) $ in Millions	Dec. 31, 2020 USD ($)
Dividend and Other Capital-Related Restrictions [Abstract]
Minimum calendar year average equity to total capitalization ratio authorized by state commission	52.50%
Equity to total capitalization ratio	52.80%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 7

The minimum calendar year average ratio of equity to total capitalization authorized by the state commission to be maintained by the entity in order to pay a maximum stated amount of dividends.

No definition available.

Name:	nspw_MinimumCalendarYearAverageEquityToTotalCapitalizationRatioAuthorizedByStateCommission
Namespace Prefix:	nspw_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant