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Employee and Retiree Benefit Plans (Tables)
 6 Months Ended
Jun. 30, 2024
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Expense
The table that follows provides the components of net periodic benefit expense for the three-month and six-month periods ended June 30, 2024 and 2023.
Three Months Ended June 30,
Pension BenefitsOther Postretirement Benefits
(Thousands of dollars)2024202320242023
Service cost$1,706 $1,650 $135 $132 
Interest cost8,393 8,564 782 874 
Expected return on plan assets(8,359)(8,254)– – 
Estimated defined contribution provision54 54 – – 
Amortization of prior service cost (credit)579 155 (133)(133)
Recognized actuarial loss (gain)2,361 2,414 (812)(767)
Total net periodic benefit expense$4,734 $4,583 $(28)$106 
Six Months Ended June 30,
Pension BenefitsOther Postretirement Benefits
(Thousands of dollars)2024202320242023
Service cost$3,412 $3,300 $270 $264 
Interest cost16,784 17,071 1,564 1,748 
Expected return on plan assets(16,716)(16,448)– – 
Estimated defined contribution provision109 108 – – 
Amortization of prior service cost (credit)1,158 310 (266)(266)
Recognized actuarial loss (gain)4,721 4,815 (1,624)(1,548)
         Total net periodic benefit expense$9,468 $9,156 $(56)$198