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Retirement Benefits
 9 Months Ended
Sep. 25, 2020
Defined Benefit Plan [Abstract]  
Retirement Benefits Retirement Benefits
The components of net periodic benefit cost for retirement benefit plans were as follows (in thousands):
 Three Months EndedNine Months Ended
 September 25,
2020		September 27,
2019		September 25,
2020		September 27,
2019
Pension Benefits
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
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