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Retirement Plans and Other Postretirement Benefits (Tables)
 12 Months Ended
Dec. 31, 2023
Retirement Benefits [Abstract]  
Schedule of Net Periodic Benefit Costs and the Portion of these Costs Charged to Expense (Including Administrative Costs and Excluding Amounts Capitalized as Overhead Construction, Billed to Electric Plant Participants or Charged or Amortized to the Regulatory Asset)
The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction or billed to electric plant participants) (dollars in thousands):
 Pension PlansOther Benefits Plans
 202320222021202320222021
Service cost-benefits earned during the period$39,461 $55,473 $61,236 $8,567 $16,470 $17,796 
Non-service costs (credits):
Interest cost on benefit obligation153,561 107,492 98,566 22,509 17,491 16,513 
Expected return on plan assets(182,938)(185,775)(202,628)(43,486)(46,042)(41,444)
Amortization of:      
Prior service credit (a)— — — (37,789)(37,789)(37,705)
Net actuarial (gain)/loss38,420 17,515 15,948 (9,614)(12,835)(10,093)
Net periodic benefit cost/(benefit)$48,504 $(5,295)$(26,878)$(59,813)$(62,705)$(54,933)
Portion of cost/(benefit) charged to expense$27,029 $(16,431)$(32,743)$(43,408)$(45,042)$(38,657)
(a)    Prior-service costs or credits reflect the impact of modifications to the pension or postretirement plan benefits. The impact of these modifications is amortized over a period which reflects the demographics of the impacted population. In 2014, Pinnacle West made changes to the postretirement benefits offered to Medicare eligible retirees which resulted in prior-service credits. We have been amortizing these prior-serviced credits since 2015 with the last full-year amortization occurring in 2024.
Schedule of Changes in the Benefit Obligations and Funded Status
The following table shows the plans’ changes in the benefit obligations and funded status (dollars in thousands):
 Pension PlansOther Benefits Plans
 2023202220232022
Change in Benefit Obligation    
Benefit obligation at January 1$2,809,529 $3,716,824 $409,461 $591,841 
Service cost39,461 55,473 8,567 16,470 
Interest cost153,561 107,492 22,509 17,491 
Benefit payments(210,737)(212,565)(30,784)(30,913)
Actuarial (gain) loss116,249 (857,695)20,681 (185,428)
Benefit obligation at December 312,908,063 2,809,529 430,434 409,461 
Change in Plan Assets    
Fair value of plan assets at January 12,829,485 3,812,041 652,287 872,435 
Actual return/(loss) on plan assets199,098 (787,874)67,317 (193,807)
Benefit payments(193,034)(194,682)(23,110)(26,341)
Fair value of plan assets at December 312,835,549 2,829,485 696,494 652,287 
Funded/(Underfunded) Status at December 31$(72,514)$19,956 $266,060 $242,826 
Schedule of Projected Benefit Obligation and the Accumulated Benefit Obligation for Pension Plans with an Accumulated Obligation in Excess of Plan Assets
The following table shows information for pension plans with an accumulated obligation in excess of plan assets (dollars in thousands):
As of December 31,
 20232022
Accumulated benefit obligation$123,701 $126,759 
Fair value of plan assets— — 
The following table shows information for pension plans with a projected benefit obligation in excess of plan assets (dollars in thousands):
As of December 31,
 20232022
Projected benefit obligation$129,891 $133,818 
Fair value of plan assets— — 
Schedule of Amounts Recognized on the Consolidated Balance Sheets
The following table shows the amounts recognized on the Consolidated Balance Sheets (dollars in thousands):
 Pension PlansOther Benefits Plans
 2023202220232022
Noncurrent asset$57,378 $153,773 $266,060 $242,826 
Current liability(17,190)(17,531)— — 
Noncurrent liability(112,702)(116,286)— — 
Net amount recognized (funded status)$(72,514)$19,956 $266,060 $242,826 
Schedule of Accumulated Other Comprehensive Loss
The following table shows the details related to accumulated other comprehensive loss (gain) as of December 31, 2023, and 2022 (dollars in thousands): 
 Pension PlansOther Benefits Plans
 2023202220232022
Net actuarial loss (gain)$743,003 $681,335 $(188,630)$(195,095)
Prior service credit— — (39,054)(76,843)
APS’s portion recorded as a regulatory (asset) liability(696,476)(637,656)226,726 270,604 
Income tax expense (benefit)(11,506)(10,797)691 784 
Accumulated other comprehensive loss (gain)$35,021 $32,882 $(267)$(550)
Schedule of Weighted-Average Assumptions Used for Both the Pension and Other Benefits to Determine Benefit Obligations and Net Periodic Benefit Costs
The following table shows the weighted-average assumptions used for both the pension and other benefits to determine benefit obligations and net periodic benefit costs:
 Benefit Obligations
As of December 31,		Benefit Costs
Year Ended December 31,
 20232022202320222021
Discount rate – pension plans5.21 %5.56 %5.56 %2.92 %2.53 %
Discount rate – other benefits plans5.23 %5.58 %5.58 %2.98 %2.63 %
Rate of compensation increase4.52 %4.57 %4.57 %4.00 %4.00 %
Expected long-term return on plan assets - pension plansN/AN/A6.70 %5.00 %5.30 %
Expected long-term return on plan assets - other benefit plansN/AN/A6.80 %5.35 %4.90 %
Initial healthcare cost trend rate (pre-65 participants)6.25 %6.50 %6.50 %6.00 %6.50 %
Ultimate healthcare cost trend rate (pre-65 participants)4.75 %4.75 %4.75 %4.75 %4.75 %
Number of years to ultimate trend rate (pre-65 participants)56534
Initial and ultimate healthcare cost trend rate (post-65 participants)2.00 %2.00 %2.00 %2.00 %2.00 %
Interest crediting rate – cash balance pension plans4.54 %4.50 %4.50 %4.50 %4.50 %
Schedule of Fair Value of Pension Plan and Other Postretirement Benefit Plan Assets, by Asset Category
Based on the IPS, the target and actual allocation for the pension plan at December 31, 2023, are as follows:
 Target AllocationActual Allocation
Long-term fixed income assets80 %78 %
Return-seeking assets20 %22 %
Total100 %100 %

The permissible range is within +/-5% of the target allocation shown in the above table, and also considers the plan’s funded status.

The following table presents the additional target allocations, as a percent of total pension plan assets, for the return-seeking assets:
Target Allocation
Equities in US and other developed markets12 %
Equities in emerging markets4 %
Alternative investments4 %
Total20 %
The following table presents the actual allocations of the investment for the other postretirement benefit plan at December 31, 2023:
Actual Allocation
Long-term fixed income assets62 %
Return-seeking assets38 %
Total100 %
The fair value of Pinnacle West’s pension plan and other postretirement benefit plan assets at December 31, 2023, by asset category, are as follows (dollars in thousands):
 
 Level 1Level 2Other (a)Total
Pension Plan:   
Fixed income securities:   
Corporate$— $1,415,346 $— $1,415,346 
U.S. Treasury622,273 — — 622,273 
Other (b)— 135,184 — 135,184 
Common stock equities (c)150,657 — — 150,657 
Mutual funds (d)112,791 — — 112,791 
Common and collective trusts:
Equities— — 192,945 192,945 
Real estate— — 140,613 140,613 
Short-term investments and other (e)— — 65,740 65,740 
Total$885,721 $1,550,530 $399,298 $2,835,549 
Other Benefits:    
Fixed income securities:   
Corporate$— $189,902 $— $189,902 
U.S. Treasury207,665 — — 207,665 
Other (b)— 8,372 — 8,372 
Common stock equities (c)139,952 — — 139,952 
Mutual funds (d)22,256 — — 22,256 
Common and collective trusts:   
Equities— — 81,724 81,724 
Real estate— — 20,001 20,001 
Short-term investments and other (e)21,146 — 5,476 26,622 
Total$391,019 $198,274 $107,201 $696,494 
(a)These investments primarily represent assets valued using NAV as a practical expedient and have not been classified in the fair value hierarchy.
(b)This category consists primarily of debt securities issued by municipalities and asset backed securities.
(c)This category primarily consists of U.S. common stock equities.
(d)These funds invest in international common stock equities.
(e)This category includes plan receivables and payables.
The fair value of Pinnacle West’s pension plan and other postretirement benefit plan assets at December 31, 2022, by asset category, are as follows (dollars in thousands):
 Level 1Level 2Other (a)Total
Pension Plan:   
Cash and cash equivalents$1,252 $— $— $1,252 
Fixed income securities:   
Corporate— 1,374,810 — 1,374,810 
U.S. Treasury635,245 — — 635,245 
Other (b)— 131,999 — 131,999 
Common stock equities (c)155,231 — — 155,231 
Mutual funds (d)101,557 — — 101,557 
Common and collective trusts:
   Equities— — 181,912 181,912 
   Real estate— — 174,228 174,228 
Partnerships— — 13,359 13,359 
Short-term investments and other (e)— — 59,892 59,892 
Total $893,285 $1,506,809 $429,391 $2,829,485 
Other Benefits:    
Cash and cash equivalents$204 $— $— $204 
Fixed income securities:   
Corporate— 166,879 — 166,879 
U.S. Treasury221,936 — — 221,936 
Other (b)— 7,321 — 7,321 
Common stock equities (c)127,493 — — 127,493 
Mutual funds (d)18,824 — — 18,824 
Common and collective trusts:
   Equities— — 73,956 73,956 
   Real estate— — 23,541 23,541 
Short-term investments and other (e)3,274 — 8,859 12,133 
Total $371,731 $174,200 $106,356 $652,287 
(a)These investments primarily represent assets valued using NAV as a practical expedient and have not been classified in the fair value hierarchy.
(b)This category consists primarily of debt securities issued by municipalities.
(c)This category primarily consists of U.S. common stock equities.
(d)These funds invest in U.S. and international common stock equities.
(e)This category includes plan receivables and payables.
Schedule of Estimated Future Benefit Payments, which Reflect Estimated Future Employee Service, for the Next Five Years and the Succeeding Five Years Thereafter
Benefit payments, which reflect estimated future employee service, for the next five years and the succeeding five years thereafter, are estimated to be as follows (dollars in thousands):
YearPension PlansOther Benefits Plans
2024$244,772 $31,024 
2025226,748 30,446 
2026229,322 30,396 
2027226,906 30,024 
2028229,397 29,741 
Years 2029-20331,136,944 149,312