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Retirement Benefits (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 25, 2020
Sep. 27, 2019
Sep. 25, 2020
Sep. 27, 2019
Pension        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 2,392 $ 2,161 $ 7,007 $ 5,805
Interest cost 3,351 3,711 9,985 11,021
Expected return on assets (4,706) (4,289) (14,107) (12,864)
Amortization and other 2,777 3,095 8,015 6,912
Net periodic benefit cost 3,814 4,678 10,900 10,874
Postretirement Medical        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 152 136 457 409
Interest cost 254 291 762 872
Amortization 183 68 550 205
Net periodic benefit cost $ 589 $ 495 $ 1,769 $ 1,486