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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
 3 Months Ended 6 Months Ended
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Non-service costs        
Non-service costs $ 5,306 $ 2,258 $ 10,267 $ 4,836
Pension benefits        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 3,166 4,532 6,093 8,388
Non-service costs        
Interest cost 7,906 6,778 16,299 12,841
Expected return on plan assets (7,947) (10,519) (16,263) (20,843)
Amortization of net actuarial losses (gains) (71) 1,257 (195) 2,046
Amortization of prior service credits (373) (389) (746) (792)
Impact of regulatory accounts 4,588 4,664 8,683 11,002
Non-service costs 4,103 1,791 7,778 4,254
Net benefit cost 7,269 6,323 13,871 12,642
OPEB        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 902 1,554 1,891 3,109
Non-service costs        
Interest cost 2,891 2,282 6,329 4,641
Expected return on plan assets (2,331) (2,841) (5,077) (5,682)
Amortization of net actuarial losses (gains) (561) (86) (1,122) (172)
Amortization of prior service credits (213) 6 (426) 12
Impact of regulatory accounts 1,417 1,106 2,785 1,783
Non-service costs 1,203 467 2,489 582
Net benefit cost $ 2,105 $ 2,021 $ 4,380 $ 3,691