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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, 2020
Dec. 31, 2019
Non-service costs    
Defined Benefit Plan, Non-service Costs, Total $ 14,072 $ 17,332
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 15,450 12,351
Non-service costs    
Interest cost 19,281 20,222
Expected return on plan assets (26,285) (20,485)
Amortization of net actuarial loss 5,430 3,530
Amortization of prior service credits (1,609) (784)
Amortization of regulatory assets/liabilities 16,272 12,082
Defined Benefit Plan, Non-service Costs, Total 13,089 14,565
Net benefit cost 28,539 26,916
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 6,175 4,587
Non-service costs    
Interest cost 7,695 7,575
Expected return on plan assets (8,748) (6,725)
Amortization of net actuarial loss 509 (409)
Amortization of prior service credits 0 (208)
Amortization of regulatory assets/liabilities 1,527 2,534
Defined Benefit Plan, Non-service Costs, Total 983 2,767
Net benefit cost $ 7,158 $ 7,354