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Pension and other post-retirement benefits - Weighted Average Assumptions Used to Determine Net Benefit Obligation (Detail)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Pension benefits    
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 2.94% 2.49%
Interest crediting rate (for cash balance plans) 4.00% 4.15%
Rate of compensation increase 4.00% 4.00%
OPEB    
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 2.92% 2.58%
Health care cost trend rate    
Before age 65 5.875% 6.00%
Age 65 and after 5.875% 6.00%
Assumed ultimate medical inflation rate 4.75% 4.75%
Year in which ultimate rate is reached 2031 2031