Benefit Plans and Other Postretirement Benefits (Tables)

12 Months Ended

Dec. 31, 2023

Benefit Plans and Other Postretirement Benefits [Abstract]

Funded Status of Plans

	Pension Benefits				Postretirement Benefits
(Millions of Dollars)	2023		2022		2023		2022
Change in Benefit Obligation:
Obligation at Jan. 1	$	112	$	141	$	8	$	11
Service cost	4		5		—		—
Interest cost	6		4		1		1
Actuarial (gain) loss	1		(21)		—		(3)
Benefit payments	(11)		(17)		(1)		(1)
Obligation at Dec. 31	$	112	$	112	$	8	$	8
Change in Fair Value of Plan Assets:
Fair value of plan assets at Jan. 1	$	97	$	137	$	—	$	—
Actual return on plan assets	5		(24)		—		—
Employer contributions	4		1		1		1
Benefit payments	(11)		(17)		(1)		(1)
Fair value of plan assets at Dec. 31	$	95	$	97	$	—	$	—
Funded status of plans at Dec. 31	$	(17)	$	(15)	$	(8)	$	(8)
Amounts recognized in the Consolidated Balance Sheet at Dec. 31:
Current liabilities	$	—	$	—	$	(1)	$	—
Noncurrent liabilities	(17)		(15)		(7)		(8)
Net amounts recognized	$	(17)	$	(15)	$	(8)	$	(8)

	Pension Benefits				Postretirement Benefits
Significant Assumptions Used to Measure Benefit Obligations:	2023		2022		2023		2022
Discount rate for year-end valuation	5.49	%	5.80	%	5.54	%	5.80	%
Expected average long-term increase in compensation level	4.25	%	4.25	%	N/A		N/A
Mortality table	Pri-2012		Pri-2012		Pri-2012		Pri-2012
Health care costs trend rate — initial: Pre-65	N/A		N/A		6.50	%	6.50	%
Health care costs trend rate — initial: Post-65	N/A		N/A		5.50	%	5.50	%
Ultimate trend assumption — initial: Pre-65	N/A		N/A		4.50	%	4.50	%
Ultimate trend assumption — initial: Post-65	N/A		N/A		4.50	%	4.50	%
Years until ultimate trend is reached	N/A		N/A		6		7

Components of Net Periodic Benefit Costs

	Pension Benefits									Postretirement Benefits
(Millions of Dollars)	2023			2022			2021			2023			2022			2021
Service cost	$	4		$	5		$	5		$	—		$	—		$	—
Interest cost	6			4			4			1			1			1
Expected return on plan assets	(8)			(8)			(8)			—			—			—
Amortization of net loss	2			3			5			—			—			—
Settlement charge (a)	—			6			5			—			—			—
Net periodic pension cost	$	4		$	10		$	11		$	1		$	1		$	1
Effects of regulation	—			(2)			(3)			—						—
Net benefit cost recognized for financial reporting	$	4		$	8		$	8		$	1		$	1		$	1
Significant Assumptions Used to Measure Costs:
Discount rate	5.80		%	3.08		%	2.71		%	5.80		%	3.09		%	2.65		%
Expected average long-term increase in compensation level	4.25			3.75			3.75			—			—			—
Expected average long-term rate of return on assets	7.25			6.60			6.60			5.00			4.10			4.10

(a)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. There were no settlement charges recorded to the qualified pension plans in 2023. In 2022, as a result of lump-sum distributions during the plan years, NSP-Wisconsin recorded a total pension settlement charge of $5 million, of which $1 million was recorded in the income statement.

Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost

	Pension Benefits				Postretirement Benefits
(Millions of Dollars)	2023		2022		2023		2022
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost:
Net loss	$	48	$	45	$	3	$	4
Prior service credit	—		—		—		—
Total	$	48	$	45	$	3	$	4
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates:
Current regulatory assets	$	2	$	1			$	—
Noncurrent regulatory assets	46		44		3		4
Total	$	48	$	45	$	3	$	4

Target Asset Allocations and Plan Assets Measured at Fair Value

For each of the fair value hierarchy levels, NSP-Wisconsin’s pension plan assets measured at fair value:

	Dec. 31, 2023 (a)										Dec. 31, 2022 (a)
(Millions of Dollars)	Level 1		Level 2		Level 3		Measured at NAV		Total		Level 1		Level 2		Level 3		Measured at NAV		Total
Cash equivalents	$	8	$	—	$	—	$	—	$	8	$	4	$	—	$	—	$	—	$	4
Commingled funds	19		—		—		45		64		35		—		—		34		69
Debt securities	—		22		—		—		22		—		22		—		—		22
Equity securities	1		—		—		—		1		2		—		—		—		2
Total	$	28	$	22	$	—	$	45	$	95	$	41	$	22	$	—	$	34	$	97

(a)See Note 8 for further information on fair value measurement inputs and methods.

NSP-Wisconsin has immaterial postretirement benefit plan assets that were measured at fair value at Dec. 31, 2023 and 2022.

Immaterial assets were transferred in or out of Level 3 for 2023. No assets were transferred in or out of Level 3 for 2022.

Target asset allocations:

	Pension Benefits				Postretirement Benefits
	2023		2022		2023		2022
Long-duration fixed income and interest rate swap securities	38	%	38	%	—	%	—	%
Domestic and international equity securities	31		33		9		16
Alternative investments	20		18		13		12
Short-to-intermediate fixed income securities	9		9		77		71
Cash	2		2		1		1
Total	100	%	100	%	100	%	100	%

The asset allocations above reflect target allocations approved in the calendar year to take effect in the subsequent year.

Projected Benefit Payments for the Pension and Postretirement Benefit Plans

(Millions of Dollars)	Projected Pension Benefit Payments		Net Projected Postretirement Health Care Benefit Payments (a)
2024	$	17	$	1
2025	8		1
2026	8		1
2027	9		1
2028	10		1
2029-2033	51		3

(a)Amount is reported net of expected Medicare Part D subsidies, which are immaterial.