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Retirement Benefits (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 26, 2015
Jun. 27, 2014
Jun. 26, 2015
Jun. 27, 2014
Pension [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 1,907 $ 1,697 $ 4,003 $ 3,439
Interest cost 3,605 3,940 7,380 8,076
Expected return on assets (4,659) (5,211) (9,576) (10,630)
Amortization and other 2,426 1,355 4,779 2,688
Net periodic benefit cost 3,279 1,781 6,586 3,573
Postretirement Medical [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 150 125 300 250
Interest cost 227 278 453 555
Amortization (101) (126) (202) (254)
Net periodic benefit cost $ 276 $ 277 $ 551 $ 551