Pension and other postretirement benefits - net benefit cost of pension and postretirement benefit plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,768	$ 1,500	$ 2,887
Interest cost	16,000	14,816	15,137
Expected return on plan assets	(27,426)	(29,666)	(26,809)
Net amortization and deferral	7,013	6,777	6,734
Curtailments	0	0	0
Settlements	0	486	0
Net benefit expense (income)	(1,645)	(6,087)	(2,051)
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	9	50	279
Interest cost	1,391	1,389	1,577
Expected return on plan assets	0	0	0
Net amortization and deferral	(1)	136	275
Curtailments	0	677	0
Settlements	0	0	0
Net benefit expense (income)	$ 1,399	$ 2,252	$ 2,131