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Pension and other post-retirement benefits - Weighted Average Assumptions Used to Determine Net Benefit Cost (Detail)
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Pension benefits    
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 2.94% 2.49%
Expected return on assets 6.19% 6.20%
Rate of compensation increase 3.91% 3.99%
OPEB    
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 3.00% 2.58%
Expected return on assets 6.48% 4.79%
Health care cost trend rate    
Before Age 65 5.88% 5.12%
Age 65 and after 5.88% 5.12%
Assumed ultimate medical inflation rate 4.75% 4.05%
Year in which ultimate rate is reached 2031 2031