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Benefit Plans and Other Postretirement Benefits (Tables)
 12 Months Ended
Dec. 31, 2025
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, 2025 (a)
Dec. 31, 2024 (a)
(Millions of Dollars)Level 1Level 2Level 3Measured at NAVTotalLevel 1Level 2Level 3Measured at NAVTotal
Cash equivalents$18 $— $— $— $18 $19 $— $— $— $19 
Commingled funds (b)
— — — 165 165 — — — 158 158 
Debt securities— 125 — — 125 — 113 1 — 114 
Equity securities3 — — — 3 4 — — — 4 
Partnerships (b)
— — — 101 101 — — — 101 101 
Other— 1 — — 1 — 1 — — 1 
Total$21 $126 $— $266 $413 $23 $114 $1 $259 $397 
(a)See Note 8 for further information on fair value measurement inputs and methods.
(b)Prior period amounts have been reclassified to conform with current year presentation.
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, 2025 (a)
Dec. 31, 2024 (a)
(Millions of Dollars)Level 1Level 2Level 3Measured at NAVTotalLevel 1Level 2Level 3Measured at NAVTotal
Cash equivalents$4 $— $— $— $4 $4 $— $— $— $4 
Insurance contracts— 4 — — 4 — 4 — — 4 
Commingled funds (b)
— — — 7 7 — — — 2 2 
Debt securities— 17 — — 17 — 21 — — 21 
Partnerships (b)
— — — 5 5 — — — 5 5 
Total$4 $21 $— $12 $37 $4 $25 $— $7 $36 
(a)See Note 8 for further information on fair value measurement inputs and methods.
(b)Prior period amounts have been reclassified to conform with current year presentation.
Target asset allocations:
Pension BenefitsPostretirement Benefits
2025202420252024
Long-duration fixed income securities38 %38 %— %— %
Domestic and international equity securities30 31 25 25 
Alternative investments19 20 13 11 
Short-to-intermediate fixed income securities11 9 61 61 
Cash2 2 1 3 
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)2025202420252024
Change in Benefit Obligation:
Obligation at Jan. 1$409 $425 $26 $26 
Service cost7 7 1 1 
Interest cost23 22 1 1 
Actuarial loss (gain)12 (12)4 1 
Plan participants’ contributions— — — 1 
Benefit payments(31)(33)(2)(4)
Obligation at Dec. 31$420 $409 $30 $26 
Change in Fair Value of Plan Assets:
Fair value of plan assets at Jan. 1$397 $420 $36 $36 
Actual return on plan assets44 7 3 2 
Employer contributions3 3 — 1 
Plan participants’ contributions— — — 1 
Benefit payments(31)(33)(2)(4)
Fair value of plan assets at Dec. 31$413 $397 $37 $36 
Funded status of plans at Dec. 31$(7)$(12)$7 $10 
Amounts recognized in the Balance Sheet at Dec. 31:
Noncurrent assets— — 7 10 
Noncurrent liabilities(7)(12)— — 
Net amounts recognized$(7)$(12)$7 $10 
Pension BenefitsPostretirement Benefits
Significant Assumptions Used to Measure Benefit Obligations:2025202420252024
Discount rate for year-end valuation5.78 %5.88 %5.66 %5.88 %
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 %7.00 %
Health care costs trend rate — initial: Post-65N/AN/A7.50 %7.50 %
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/A89
Accumulated benefit obligation for the pension plan was $393 million and $383 million as of Dec. 31, 2025 and 2024, respectively.
Components of Net Periodic Benefit Costs
Net Periodic Benefit Cost — Net periodic benefit cost, other than the service cost component, is included in other income, net in the statements of income.
Components of net periodic benefit cost and amounts recognized in other comprehensive income and regulatory assets and liabilities:
Pension BenefitsPostretirement Benefits
(Millions of Dollars)202520242023202520242023
Service cost$7 $7 $7 $1 $1 $1 
Interest cost23 22 22 1 1 1 
Expected return on plan assets(32)(32)(32)(2)(2)(1)
Amortization of net loss (gain)3 3 2 (1)— — 
Net periodic pension cost1 — (1)(1)— 1 
Effects of regulation2 (1)5 — — — 
Net benefit cost recognized for financial reporting$3 $(1)$4 $(1)$— $1 
Significant Assumptions Used to Measure Costs:
Discount rate5.88 %5.49 %5.80 %5.88 %5.54 %5.80 %
Expected average long-term increase in compensation level4.25 4.25 4.25 — — — 
Expected average long-term rate of return on assets7.03 6.97 6.96 6.25 5.00 5.00 
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)
2026$31 $3 
202731 3 
202832 3 
202932 3 
203033 3 
2031-2035161 11 
(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)2025202420252024
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost:
Net loss (gain)$162 $165 $(15)$(19)
Prior service credit(1)(1)— — 
Total$161 $164 $(15)$(19)
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$5 $4 $— $— 
Noncurrent regulatory assets156 160 — — 
Current regulatory liabilities— — — (1)
Noncurrent regulatory liabilities— — (15)(18)
Total$161 $164 $(15)$(19)
Measurement dateDec. 31, 2025Dec. 31, 2024Dec. 31, 2025Dec. 31, 2024