Pension and other postretirement benefits - Net benefit cost of pension and postretirement benefit plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Settlements	$ 45,244	$ 0	$ 0
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,435	1,346	1,467
Interest cost	17,297	10,776	9,272
Expected return on plan assets	(25,277)	(25,776)	(30,726)
Net amortization and deferral	8,536	7,900	8,589
Settlements	45,244	0	0
Net benefit expense (income)	47,235	(5,754)	(11,398)
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0
Interest cost	824	477	418
Expected return on plan assets	0	0	0
Net amortization and deferral	(1,564)	(1,258)	(1,058)
Settlements	0	0	0
Net benefit expense (income)	$ (740)	$ (781)	$ (640)