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Retirement Benefits (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 26, 2021
Mar. 27, 2020
Pension    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 2,721 $ 2,422
Interest cost 2,981 3,407
Expected return on assets (5,130) (4,750)
Amortization and other 2,551 2,721
Net periodic benefit cost 3,123 3,800
Postretirement Medical    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 175 175
Interest cost 250 248
Amortization 250 177
Net periodic benefit cost $ 675 $ 600