Benefit Plans and Other Postretirement Benefits, Postretirement Benefit Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
		Jan. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Funded Status of Plans at Dec. 31 [Abstract]
Noncurrent liabilities			$ (28)	$ (33)
Pension Plan [Member]
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1		$ 154	144	140
Service cost			4	4	$ 5
Interest cost			5	6	5
Actuarial loss			13	8
Benefit payments			(12)	(14)
Obligation at Dec. 31			154	144	140
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		140	124	110
Actual return (loss) on plan assets			21	21
Employer contributions			7	7
Benefit payments			(12)	(14)
Fair value of plan assets at Dec. 31			140	124	110
Funded Status of Plans at Dec. 31 [Abstract]
Funded status			(14)	(20)
Current liabilities			0	0
Noncurrent liabilities			(14)	(20)
Net postretirement amounts recognized on consolidated balance sheets			(14)	(20)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]
Net loss			60	64
Prior service (credit) cost			0	0
Total			60	64
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Current regulatory assets			4	4
Noncurrent regulatory assets			56	60
Total			$ 60	$ 64
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)			2.71%	3.49%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase			3.75%	3.75%
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			$ 4	$ 4	5
Interest cost			5	6	5
Expected return on plan assets			(8)	(8)	(9)
Amortization of net loss			5	4	6
Settlement charge	[1]		0	0	7
Net periodic benefit cost			$ 6	$ 6	$ 14
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)			3.49%	4.31%	3.63%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase			3.75%	3.75%	3.75%
Expected average long-term rate of return on assets (as a percent)			7.10%	7.10%	7.10%
Defined Benefit Plan Credits (Costs) Not Recognized Due To Effects of Regulation			$ (7)	$ 0	$ (3)
Net benefit cost recognized for financial reporting			13	6	11
Other Postretirement Benefits Plan [Member]
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1		13	13	13
Service cost			0	0	0
Interest cost			1	1	1
Actuarial loss			0	0
Benefit payments			(1)	(1)
Obligation at Dec. 31			13	13	13
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		0	0	0
Actual return (loss) on plan assets			0	0
Employer contributions			1	1
Benefit payments			(1)	(1)
Fair value of plan assets at Dec. 31			0	0	0
Funded Status of Plans at Dec. 31 [Abstract]
Funded status			(13)	(13)
Current liabilities			(1)	(1)
Noncurrent liabilities			(12)	(12)
Net postretirement amounts recognized on consolidated balance sheets			(13)	(13)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]
Net loss			7	7
Prior service (credit) cost			(1)	(1)
Total			6	6
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Current regulatory assets			0	0
Noncurrent regulatory assets			6	6
Total			$ 6	$ 6
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)			2.65%	3.47%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Pre-65			5.50%	6.00%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Post-65			5.00%	5.10%
Ultimate health care trend assumption rate (as a percent)			4.50%	4.50%
Period until ultimate trend rate is reached (in years)			5	3
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			$ 0	$ 0	0
Interest cost			1	1	1
Expected return on plan assets			0	0	0
Amortization of net loss			0	0	0
Settlement charge	[1]		0	0	0
Net periodic benefit cost			$ 1	$ 1	$ 1
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)			3.47%	4.32%	3.62%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase			0.00%	0.00%	0.00%
Expected average long-term rate of return on assets (as a percent)			4.50%	4.50%	5.30%
Defined Benefit Plan Credits (Costs) Not Recognized Due To Effects of Regulation			$ 0	$ 0	$ 0
Net benefit cost recognized for financial reporting			$ 1	$ 1	$ 1
Subsequent Event | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension Benefits		$ 125

[1] A settlement charge is required when the amount of all lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In 2018, as a result of lump-sum distributions during the 2018 plan year, NSP-Wisconsin recorded a total pension settlement charge of $7 million in 2018, a total of $2 million of that amount was recorded in the income statement in 2018. There were no settlement charges recorded to the qualified pension plans in 2020 and 2019.