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Pension and other postretirement benefits (Tables)
 9 Months Ended
Sep. 30, 2018
Retirement Benefits [Abstract]  
Schedule of Net Benefit Cost of Pension and Postretirement Benefit Plans
Net pension and other postretirement benefits expense (income) consist of the following:
 
Pension
Three Months Ended
 
Other Postretirement Benefits
Three Months Ended
 
Pension
Nine Months Ended
 
Other Postretirement Benefits
Nine Months Ended
 
September 30, 2018
 
October 1, 2017
 
September 30, 2018
 
October 1, 2017
 
September 30, 2018
 
October 1, 2017
 
September 30, 2018
 
October 1, 2017
 
(Dollars in thousands)
Service cost
$
380

 
$
725

 
$
(66
)
 
$
61

 
$
1,135

 
$
2,162

 
$
38

 
$
210

Interest cost
3,681

 
3,773

 
285

 
426

 
11,119

 
11,347

 
1,042

 
1,183

Expected return on plan assets
(7,423
)
 
(6,607
)
 
—

 
—

 
(22,260
)
 
(20,100
)
 
—

 
—

Net amortization and deferral
1,687

 
1,668

 
(23
)
 
75

 
5,090

 
5,049

 
101

 
206

Net benefits expense (income)
$
(1,675
)
 
$
(441
)
 
$
196

 
$
562

 
$
(4,916
)
 
$
(1,542
)
 
$
1,181

 
$
1,599