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Pension and other postretirement benefits - Net benefit cost of pension and postretirement benefit plans (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 26, 2016
Jun. 28, 2015
Jun. 26, 2016
Jun. 28, 2015
Pension        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 655 $ 469 $ 1,307 $ 940
Interest cost 3,929 4,485 7,849 8,971
Expected return on plan assets (6,199) (6,427) (12,397) (12,852)
Net amortization and deferral 1,577 1,528 3,156 3,058
Net benefits expense (income) (38) 55 (85) 117
Other Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 111 107 222 214
Interest cost 407 511 813 1,023
Expected return on plan assets 0 0 0 0
Net amortization and deferral 57 78 114 155
Net benefits expense (income) $ 575 $ 696 $ 1,149 $ 1,392