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Note Pension and postretirement benefits (Components of net periodic postretirement benefit cost) (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Pension and Other Postretirement Benefits Disclosure [Line Items]        
Amortization of prior service credit of pension and postretirement benefit plans $ (868) $ (950) $ (2,603) $ (2,850)
Amortization of net losses (5,386) (5,606) (16,157) (16,819)
Other Postretirement Benefit Plans Defined Benefit [Member]        
Pension and Other Postretirement Benefits Disclosure [Line Items]        
Service cost 257 256 771 769
Interest cost 1,390 1,426 4,171 4,277
Amortization of prior service credit of pension and postretirement benefit plans (868) (950) (2,603) (2,850)
Amortization of net losses 321 142 962 426
Total postretirement cost $ 1,100 $ 874 $ 3,301 $ 2,622