Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) $ in Millions		1 Months Ended	12 Months Ended
		Jan. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) Plan	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Settlement Charge Recognized in Operating and Maintenance Expenses			$ 191	$ 201	$ 203
Significant Assumptions Used to Measure Costs [Abstract]
Liability, Defined Benefit Plan, Noncurrent			(28)	(33)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation at Dec. 31			141	132
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)
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%
Expected average long-term increase in compensation level (as a percent)			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
Net benefit cost recognized for financial reporting			$ 13	6	$ 11
Settlement Charge Recognized in Operating and Maintenance Expenses				$ 2
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%
Liability, Defined Benefit Plan, Current			$ 0	$ 0
Liability, Defined Benefit Plan, Noncurrent			(14)	(20)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position			(14)	(20)
Defined Benefit Plan Credits (Costs) Not Recognized Due To Effects of Regulation			$ 7	$ 0	$ 3
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets			7.10%	7.10%	7.10%
Pension Plan [Member] | NSP Wisconsin [Member]
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1			$ 124
Fair value of plan assets at Dec. 31				$ 124
Cash Flows [Abstract]
Payment for Pension Benefits			$ 7	7	$ 10
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Settlement charge				7
Significant Assumptions Used to Measure Costs [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets			7.10%
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)
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%
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
Net benefit cost recognized for financial reporting			$ 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%
Liability, Defined Benefit Plan, Current			$ 1	$ 1
Liability, Defined Benefit Plan, Noncurrent			(12)	(12)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position			$ (13)	$ (13)
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
Defined Benefit Plan Credits (Costs) Not Recognized Due To Effects of Regulation			$ 0	$ 0	$ 0
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets			4.50%	4.50%	5.30%
Subsequent Event | Pension Plan [Member]
Cash Flows [Abstract]
Payment for Pension Benefits		125
Subsequent Event | Pension Plan [Member] | NSP Wisconsin [Member]
Cash Flows [Abstract]
Payment for Pension Benefits		$ 5
Parent Company [Member] | Pension Plan [Member]
Cash Flows [Abstract]
Number of pension plans to which contributions were made | Plan			4

[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.