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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, 2021
Dec. 31, 2020
Non-service costs    
Non-service Costs $ 16,313 $ 14,072
Pension benefits    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 14,673 15,450
Non-service costs    
Interest cost 20,676 19,281
Expected return on plan assets (35,972) (26,285)
Amortization of net actuarial loss 13,126 5,430
Amortization of prior service credits (1,626) (1,609)
Settlement Loss Recognized 198 0
Impact of regulatory accounts 19,665 16,272
Non-service Costs 16,067 13,089
Net benefit cost 30,740 28,539
OPEB    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 7,307 6,175
Non-service costs    
Interest cost 8,048 7,695
Expected return on plan assets (10,052) (8,748)
Amortization of net actuarial loss 2,021 509
Amortization of prior service credits 11 0
Settlement Loss Recognized 0 0
Impact of regulatory accounts 218 1,527
Non-service Costs 246 983
Net benefit cost $ 7,553 $ 7,158