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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, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Non-service costs        
Non-service Costs $ 3,861 $ 3,617 $ 7,545 $ 6,973
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 4,508 4,136 8,336 7,703
Non-service costs        
Interest cost 3,511 4,112 10,217 8,903
Expected return on plan assets (6,616) (6,261) (17,780) (12,510)
Amortization of net actuarial loss 2,540 1,145 4,812 2,290
Amortization of prior service credits (406) (402) (813) (804)
Impact of regulatory accounts 4,825 4,769 11,009 8,307
Non-service Costs 3,854 3,363 7,445 6,186
Net benefit cost 8,362 7,499 15,781 13,889
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1,772 1,466 3,544 2,933
Non-service costs        
Interest cost 3,031 1,755 4,052 3,574
Expected return on plan assets (2,510) (2,193) (5,021) (4,385)
Amortization of net actuarial loss 436 (14) 873 (27)
Impact of regulatory accounts (950) 706 196 1,625
Non-service Costs 7 254 100 787
Net benefit cost $ 1,779 $ 1,720 $ 3,644 $ 3,720