Pension and other postretirement benefits - net benefit cost of pension and postretirement benefit plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,500	$ 2,887	$ 2,615
Interest cost	14,816	15,137	15,711
Expected return on plan assets	(29,666)	(26,809)	(24,786)
Net amortization and deferral	6,777	6,734	6,567
Curtailments	0	0	0
Settlements	486	0	0
Net benefit expense (income)	(6,087)	(2,051)	107
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	50	279	355
Interest cost	1,389	1,577	1,595
Expected return on plan assets	0	0	0
Net amortization and deferral	136	275	454
Curtailments	677	0	0
Settlements	0	0	0
Net benefit expense (income)	$ 2,252	$ 2,131	$ 2,404