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Retirement Plans and Other Postretirement Benefits (Tables)
3 Months Ended
Mar. 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 BenefitsOther Benefits
 Three Months Ended
March 31,
Three Months Ended
March 31,
 2023202220232022
Service cost — benefits earned during the period$10,157 $14,331 $2,120 $4,218 
Non-service costs (credits):
Interest cost on benefit obligation38,362 27,023 5,673 4,463 
Expected return on plan assets(45,561)(46,394)(10,872)(11,510)
  Amortization of:   
  Prior service credit— — (9,447)(9,447)
  Net actuarial loss (gain)9,713 4,768 (2,303)(2,982)
Net periodic cost/(benefit)$12,671 $(272)$(14,829)$(15,258)
Portion of benefit charged to expense$7,227 $(3,290)$(10,737)$(10,895)