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Retirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 27, 2024
Dec. 29, 2023
Dec. 30, 2022
Pension Benefits      
Components of net periodic benefit cost      
Service cost-benefits earned during the period $ 5,097 $ 5,729 $ 8,242
Interest cost on projected benefit obligation 9,194 16,535 10,996
Expected return on assets (10,147) (19,141) (19,754)
Amortization of prior service cost (credit) (281) 36 84
Amortization of net loss (gain) 3,255 5,999 4,701
Settlement loss (gain) 346 42,169 0
Cost of pension plans which are not significant and have not adopted ASC 715 171 368 284
Net periodic benefit cost 7,635 51,695 4,553
Postretirement Medical Benefits      
Components of net periodic benefit cost      
Service cost-benefits earned during the period 344 348 516
Interest cost on projected benefit obligation 1,148 1,165 839
Expected return on assets 0 0 0
Amortization of prior service cost (credit) 0 0 0
Amortization of net loss (gain) 0 (133) 345
Settlement loss (gain) 0 0 0
Net periodic benefit cost $ 1,492 $ 1,380 $ 1,700