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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, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Non-service costs        
Non-service costs $ 3,747 $ 520 $ 5,040 $ 328
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 2,795 3,614 6,060 7,228
Non-service costs        
Interest cost 3,670 4,459 8,429 8,390
Expected return on plan assets (3,088) (7,005) (10,211) (14,011)
Amortization of net actuarial loss 2,060 111 1,744 223
Amortization of prior service credits (412) (156) (390) (311)
Amortization of regulatory assets/liability 1,194 3,091 4,271 5,157
Non-service costs 3,424 500 3,843 (552)
Net benefit cost 6,219 4,114 9,903 6,676
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1,203 1,487 2,402 2,974
Non-service costs        
Interest cost 1,834 1,625 3,618 3,251
Expected return on plan assets (1,371) (1,849) (3,301) (3,697)
Amortization of net actuarial loss (424) (38) (476) (77)
Amortization of prior service credits (10) (65) (105) (131)
Amortization of regulatory assets/liability 294 347 1,461 1,534
Non-service costs 323 20 1,197 880
Net benefit cost $ 1,526 $ 1,507 $ 3,599 $ 3,854