Note Pension and postretirement benefits (Components of net periodic postretirement benefit cost) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension and Other Postretirement Benefits Disclosure [Line Items]
Amortization of prior service credit	$ (950)	$ (950)	$ (2,850)	$ (2,850)
Other Postretirement Benefit Plans Defined Benefit [Member]
Pension and Other Postretirement Benefits Disclosure [Line Items]
Service cost	256	289	769	867
Interest cost	1,426	1,505	4,277	4,515
Amortization of prior service credit	(950)	(950)	(2,850)	(2,850)
Amortization of net loss (gain)	142	275	426	825
Total net periodic pension cost	$ 874	$ 1,119	$ 2,622	$ 3,357