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Note Pension and postretirement benefits (Components of net periodic postretirement benefit cost) (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Pension and Other Postretirement Benefits Disclosure [Line Items]        
Amortization of prior service credit $ (950) $ (950) $ (1,900) $ (1,900)
Other Postretirement Benefit Plans Defined Benefit [Member]        
Pension and Other Postretirement Benefits Disclosure [Line Items]        
Service cost 256 289 513 578
Interest cost 1,426 1,505 2,851 3,010
Amortization of prior service credit (950) (950) (1,900) (1,900)
Amortization of net loss (gain) 142 275 284 550
Total net periodic pension cost $ 874 $ 1,119 $ 1,748 $ 2,238