Pension and other postretirement benefits - Net benefit cost of pension and postretirement benefit plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,416	$ 2,768	$ 1,500
Interest cost	12,827	16,000	14,816
Expected return on plan assets	(31,650)	(27,426)	(29,666)
Net amortization and deferral	7,447	7,013	6,777
Curtailments	0	0	0
Settlements	0	0	486
Net benefit (income) expense	(9,960)	(1,645)	(6,087)
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	9	50
Interest cost	902	1,391	1,389
Expected return on plan assets	0	0	0
Net amortization and deferral	(161)	(1)	136
Curtailments	0	0	677
Settlements	0	0	0
Net benefit (income) expense	$ 741	$ 1,399	$ 2,252