Note Pension and postretirement benefits - Additional information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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
Defined Benefit Plan Contributions By Employer	59
Reclassification Other Components From Personnel To Other Operating Expenses		$ 1,200
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
Defined Benefit Plan Contributions By Employer	1,200
Reclassification Other Components From Personnel To Other Operating Expenses		600
Service cost	$ 257	$ 256