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Note Pension and postretirement benefits - Additional information (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Other Pension Plans Defined Benefit [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined Benefit Plan Expected Future Employer Contributions Remainder Of Fiscal Year $ 235   $ 235  
Defined Benefit Plan Contributions By Employer 59      
Reclassification Other Components From Personnel To Other Operating Expenses   $ 1,200   $ 3,700
Service cost 0 0    
Other Postretirement Benefit Plans Defined Benefit [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined Benefit Plan Expected Future Employer Contributions Remainder Of Fiscal Year 6,300   6,300  
Defined Benefit Plan Contributions By Employer 1,400      
Reclassification Other Components From Personnel To Other Operating Expenses   600   1,900
Service cost $ 257 $ 256 $ 771 $ 769