Note Pension and postretirement benefits (Components of net periodic postretirement benefit cost) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension and Other Postretirement Benefits Disclosure [Line Items]
Amortization of prior service credit	$ (868)	$ (950)	$ (1,735)	$ (1,900)
Amortization of net losses	(5,385)	(5,606)	(10,771)	(11,213)
Other Postretirement Benefit Plans Defined Benefit [Member]
Pension and Other Postretirement Benefits Disclosure [Line Items]
Service cost	257	256	514	513
Interest cost	1,390	1,426	2,780	2,851
Amortization of prior service credit	(868)	(950)	(1,735)	(1,900)
Amortization of net losses	321	142	641	284
Total postretirement cost	$ 1,100	$ 874	$ 2,200	$ 1,748