Pension and other postretirement benefits - Net benefit cost of pension and postretirement benefit plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,467	$ 1,416	$ 2,768
Interest cost	9,272	12,827	16,000
Expected return on plan assets	(30,726)	(31,650)	(27,426)
Net amortization and deferral	8,589	7,447	7,013
Net benefit (income) expense	(11,398)	(9,960)	(1,645)
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	9
Interest cost	418	902	1,391
Expected return on plan assets	0	0	0
Net amortization and deferral	(1,058)	(161)	(1)
Net benefit (income) expense	$ (640)	$ 741	$ 1,399