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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, 2018
Dec. 31, 2017
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 15,481 $ 14,747
Interest cost 18,717 20,191
Expected return on plan assets (27,820) (24,842)
Amortization of net actuarial loss (gain) 1,119 1,140
Amortization of prior service credits (649) (622)
Amortization of regulatory assets/liability 9,823 13,031
Net benefit cost 16,671 23,645
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 5,791 4,838
Interest cost 6,727 6,642
Expected return on plan assets (7,451) (6,404)
Amortization of net actuarial loss (gain) (272) (230)
Amortization of prior service credits (262) (262)
Amortization of regulatory assets/liability 3,982 391
Net benefit cost $ 8,515 $ 4,975