Note Pension and postretirement benefits (Components of net periodic postretirement benefit cost) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension and Other Postretirement Benefits Disclosure [Line Items]
Amortization of prior service credit	$ (950)	$ (950)
Other Postretirement Benefit Plans Defined Benefit [Member]
Pension and Other Postretirement Benefits Disclosure [Line Items]
Service cost	256	289
Interest cost	1,426	1,505
Amortization of prior service credit	(950)	(950)
Amortization of net loss (gain)	142	275
Total net periodic pension cost	$ 874	$ 1,119