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Pension and other post-retirement 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, 2022
Dec. 31, 2021
Non-service costs    
Non-service costs $ 10,950 $ 16,313
Pension benefits    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 16,309 14,673
Non-service costs    
Interest cost 24,787 20,676
Expected return on plan assets (41,226) (35,972)
Amortization of net actuarial loss 3,452 13,126
Amortization of prior service credits (1,584) (1,626)
Amortization due to plan settlements (15) 198
Impact of regulatory accounts 22,951 19,665
Non-service costs 8,365 16,067
Net benefit cost 24,674 30,740
OPEB    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 6,277 7,307
Non-service costs    
Interest cost 9,146 8,048
Expected return on plan assets (11,359) (10,052)
Amortization of net actuarial loss (56) 2,021
Amortization of prior service credits 24 11
Amortization due to plan settlements 0 0
Impact of regulatory accounts 4,829 218
Non-service costs 2,584 246
Net benefit cost $ 8,861 $ 7,553