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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
 3 Months Ended 6 Months Ended
Jun. 30, 2022
Jun. 30, 2021
Jun. 30, 2022
Jun. 30, 2021
Non-service costs        
Non-service costs $ 2,258 $ 3,861 $ 4,836 $ 7,545
Pension benefits        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 4,532 4,508 8,388 8,336
Non-service costs        
Interest cost 6,778 3,511 12,841 10,217
Expected return on plan assets (10,519) (6,616) (20,843) (17,780)
Amortization of net actuarial loss 1,257 2,540 2,046 4,812
Amortization of prior service credits (389) (406) (792) (813)
Impact of regulatory accounts 4,664 4,825 11,002 11,009
Non-service costs 1,791 3,854 4,254 7,445
Net benefit cost 6,323 8,362 12,642 15,781
OPEB        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1,554 1,772 3,109 3,544
Non-service costs        
Interest cost 2,282 3,031 4,641 4,052
Expected return on plan assets (2,841) (2,510) (5,682) (5,021)
Amortization of net actuarial loss (86) 436 (172) 873
Amortization of prior service credits 6 0 12 0
Impact of regulatory accounts 1,106 (950) 1,783 196
Non-service costs 467 7 582 100
Net benefit cost $ 2,021 $ 1,779 $ 3,691 $ 3,644