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Benefit Plans and Other Postretirement Benefits (Tables)
 12 Months Ended
Dec. 31, 2024
Benefit Plans and Other Postretirement Benefits [Abstract]  
Target Asset Allocations and Plan Assets Measured at Fair Value
For each of the fair value hierarchy levels, SPS’ pension plan assets measured at fair value:
Dec. 31, 2024 (a)
Dec. 31, 2023 (a)
(Millions of Dollars)Level 1Level 2Level 3Measured at NAVTotalLevel 1Level 2Level 3Measured at NAVTotal
Cash equivalents$19 $— $— $— $19 $38 $— $— $— $38 
Commingled funds— — — 259 259 72 — — 190 262 
Debt securities— 113 1 — 114 — 116 1 — 117 
Equity securities4 — — — 4 5 — — — 5 
Other— 1 — — 1 — (2)— — (2)
Total$23 $114 $1 $259 $397 $115 $114 $1 $190 $420 
(a)See Note 8 for further information on fair value measurement inputs and methods.
For each of the fair value hierarchy levels, SPS’ proportionate allocation of the total postretirement benefit plan assets that were measured at fair value:
Dec. 31, 2024 (a)
Dec. 31, 2023 (a)
(Millions of Dollars)Level 1Level 2Level 3Measured at NAVTotalLevel 1Level 2Level 3Measured at NAVTotal
Cash equivalents$4 $— $— $— $4 $3 $— $— $— $3 
Insurance contracts— 4 — — 4 — 5 — — 5 
Commingled funds— — — 7 7 2 — — 7 9 
Debt securities— 21 — — 21 — 19 — — 19 
Total$4 $25 $— $7 $36 $5 $24 $— $7 $36 
(a)See Note 8 for further information on fair value measurement inputs and methods.
Target asset allocations:
Pension BenefitsPostretirement Benefits
2024202320242023
Long-duration fixed income securities38 %38 %— %— %
Domestic and international equity securities31 31 25 9 
Alternative investments20 20 11 13 
Short-to-intermediate fixed income securities9 9 61 77 
Cash2 2 3 1 
Total100 %100 %100 %100 %
Funded Status of Plans
Funded Status — Comparisons of the actuarially computed benefit obligation, changes in plan assets and funded status of the pension and postretirement health care plans for SPS are as follows:
Pension BenefitsPostretirement Benefits
(Millions of Dollars)2024202320242023
Change in Benefit Obligation:
Obligation at Jan. 1$425 $414 $26 $26 
Service cost7 7 1 1 
Interest cost22 22 1 1 
Actuarial (gain) loss(12)16 1 1 
Plan participants’ contributions— — 1 1 
Benefit payments(33)(34)(4)(4)
Obligation at Dec. 31$409 $425 $26 $26 
Change in Fair Value of Plan Assets:
Fair value of plan assets at Jan. 1$420 $417 $36 $36 
Actual return on plan assets7 37 2 3 
Employer contributions3 — 1 — 
Plan participants’ contributions— — 1 1 
Benefit payments(33)(34)(4)(4)
Fair value of plan assets at Dec. 31$397 $420 $36 $36 
Funded status of plans at Dec. 31$(12)$(5)$10 $10 
Amounts recognized in the Balance Sheet at Dec. 31:
Noncurrent assets— 1 10 10 
Noncurrent liabilities(12)(6)— — 
Net amounts recognized$(12)$(5)$10 $10 
Pension BenefitsPostretirement Benefits
Significant Assumptions Used to Measure Benefit Obligations:2024202320242023
Discount rate for year-end valuation5.88 %5.49 %5.88 %5.54 %
Expected average long-term increase in compensation level4.25 %4.25 %N/AN/A
Mortality tablePri-2012Pri-2012Pri-2012Pri-2012
Health care costs trend rate — initial: Pre-65N/AN/A7.00 %6.50 %
Health care costs trend rate — initial: Post-65N/AN/A7.50 %5.50 %
Ultimate trend assumption — initial: Pre-65N/AN/A4.50 %4.50 %
Ultimate trend assumption — initial: Post-65N/AN/A4.50 %4.50 %
Years until ultimate trend is reachedN/AN/A96
Accumulated benefit obligation for the pension plan was $383 million and $395 million as of Dec. 31, 2024 and 2023, respectively.
Components of Net Periodic Benefit Costs
Net Periodic Benefit Cost (Credit) — Net periodic benefit cost (credit), other than the service cost component, is included in other income, net in the statements of income.
Components of net periodic benefit cost (credit) and amounts recognized in other comprehensive income and regulatory assets and liabilities:
Pension BenefitsPostretirement Benefits
(Millions of Dollars)202420232022202420232022
Service cost$7 $7 $10 $1 $1 $1 
Interest cost22 22 16 1 1 1 
Expected return on plan assets(32)(32)(31)(2)(1)(2)
Amortization of net loss (gain)3 2 10 — — — 
Settlement charge (a)
— — 2 — — — 
Net periodic pension cost— (1)7 — 1 — 
Effects of regulation(1)5 (1)— — — 
Net benefit cost recognized for financial reporting$(1)$4 $6 $— $1 $— 
Significant Assumptions Used to Measure Costs:
Discount rate5.49 %5.80 %3.08 %5.54 %5.80 %3.09 %
Expected average long-term increase in compensation level4.25 4.25 3.75 — — — 
Expected average long-term rate of return on assets6.97 6.96 6.39 5.00 5.00 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 2024 or 2023. In 2022, as a result of lump-sum distributions during each plan year, SPS recorded a total pension settlement charge of $2 million. An immaterial amount was recorded in the income statement in 2022.
Projected Benefit Payments for the Pension and Postretirement Benefit Plans
Projected Benefit Payments
SPS’ projected benefit payments:
(Millions of Dollars)Projected
Pension Benefit
Payments
Net Projected
Postretirement Health Care
Benefit Payments (a)
2025$30 $2 
202630 2 
202732 2 
202832 2 
202932 2 
2030-2034158 10 
(a)Amount is reported net of expected Medicare Part D subsidies, which are immaterial.
Other Postretirement Benefits Plan [Member]  
Benefit Plans and Other Postretirement Benefits [Abstract]  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost
Pension BenefitsPostretirement Benefits
(Millions of Dollars)2024202320242023
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost:
Net loss (gain)$165 $155 $(19)$(20)
Prior service credit(1)(1)— — 
Total$164 $154 $(19)$(20)
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$4 $2 $— $— 
Noncurrent regulatory assets160 152 — — 
Current regulatory liabilities— — (1)(1)
Noncurrent regulatory liabilities— — (18)(19)
Total$164 $154 $(19)$(20)
Measurement dateDec. 31, 2024Dec. 31, 2023Dec. 31, 2024Dec. 31, 2023