Pension and other postretirement benefits - Weighted average assumptions used in determining net periodic benefit cost (Detail)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Pension
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.80%	2.50%	3.20%
Rate of return	5.60%	6.70%	7.50%
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.70%	2.30%	3.10%
Initial healthcare trend rate	6.40%	6.80%	7.00%
Ultimate healthcare trend rate	4.50%	4.50%	5.00%