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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, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Non-service costs        
Defined Benefit Plan, Non-service Costs, Total $ 3,617 $ 3,747 $ 6,973 $ 5,040
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 4,136 2,795 7,703 6,060
Non-service costs        
Interest cost 4,112 3,670 8,903 8,429
Expected return on plan assets (6,261) (3,088) (12,510) (10,211)
Amortization of net actuarial loss 1,145 2,060 2,290 1,744
Amortization of prior service credits (402) (412) (804) (390)
Amortization of regulatory assets/liabilities 4,769 1,194 8,307 4,271
Defined Benefit Plan, Non-service Costs, Total 3,363 3,424 6,186 3,843
Net benefit cost 7,499 6,219 13,889 9,903
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1,466 1,203 2,933 2,402
Non-service costs        
Interest cost 1,755 1,834 3,574 3,618
Expected return on plan assets (2,193) (1,371) (4,385) (3,301)
Amortization of net actuarial loss (14) (424) (27) (476)
Amortization of prior service credits 0 (10) 0 (105)
Amortization of regulatory assets/liabilities 706 294 1,625 1,461
Defined Benefit Plan, Non-service Costs, Total 254 323 787 1,197
Net benefit cost $ 1,720 $ 1,526 $ 3,720 $ 3,599