Pension and other postretirement benefits - Weighted average assumptions used in determining net periodic benefit cost (Detail)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension
Defined Benefit Plan Disclosure [Line Items]
Discount rate	5.10%	2.80%	2.50%
Rate of return	7.30%	5.60%	6.70%
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	5.10%	2.70%	2.30%
Initial healthcare trend rate	6.10%	6.40%	6.80%
Ultimate healthcare trend rate	4.50%	4.50%	4.50%