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Pension and other post-employment benefits - Components of Net Benefit Costs for Pension Plans and OPEB Recorded as Part of Administrative Expenses (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Non-service costs    
Non-service costs $ 19,939 $ 10,950
Pension benefits    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 11,954 16,309
Non-service costs    
Interest cost 33,687 24,787
Expected return on plan assets (31,990) (41,226)
Amortization of net actuarial losses (gains) (852) 3,452
Amortization of prior service credits (1,491) (1,584)
Amortization due to plan settlements 0 (15)
Impact of regulatory accounts 16,258 22,952
Non-service costs 15,612 8,366
Net benefit cost 27,566 24,675
OPEB    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 3,253 6,277
Non-service costs    
Interest cost 11,510 9,146
Expected return on plan assets (9,736) (11,359)
Amortization of net actuarial losses (gains) (3,559) (56)
Amortization of prior service credits (853) 24
Amortization due to plan settlements 0 0
Impact of regulatory accounts 6,965 4,829
Non-service costs 4,327 2,584
Net benefit cost $ 7,580 $ 8,861