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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, 2019
Dec. 31, 2018
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 12,351 $ 15,481
Interest cost 20,222 19,077
Expected return on plan assets (20,485) (27,820)
Amortization of net actuarial loss (gain) 3,530 1,074
Amortization of prior service credits (784) (649)
Amortization of regulatory assets/liabilities 12,082 10,584
Defined Benefit Plan, Non-service Costs, Total 14,565 2,266
Net benefit cost 26,916 17,747
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 4,587 5,791
Interest cost 7,575 6,727
Expected return on plan assets (6,725) (7,451)
Amortization of net actuarial loss (gain) (409) (272)
Amortization of prior service credits (208) (262)
Amortization of regulatory assets/liabilities 2,534 3,982
Defined Benefit Plan, Non-service Costs, Total 2,767 2,724
Net benefit cost $ 7,354 $ 8,515