Pension and other postretirement benefits - Weighted average assumptions used in determining net periodic benefit cost (Detail)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.50%	3.20%	4.30%
Rate of return	6.70%	7.50%	7.70%
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.30%	3.10%	4.20%
Initial healthcare trend rate	6.80%	7.00%	7.40%
Ultimate healthcare trend rate	4.50%	5.00%	5.00%