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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) - CAD
CAD in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost CAD 17,869 CAD 8,435
Interest cost 27,346 13,029
Expected return on plan assets (32,244) (14,854)
Amortization of net actuarial loss (gain) 1,480 1,965
Amortization of prior service credits (808) (765)
Gain on curtailments and settlements (1,394) 0
Amortization of regulatory assets/liability 15,179 4,698
Net benefit cost 27,428 12,508
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 6,280 2,916
Interest cost 8,621 3,525
Expected return on plan assets (8,312) (1,265)
Amortization of net actuarial loss (gain) (299) 80
Amortization of prior service credits (339) (347)
Gain on curtailments and settlements (6) 0
Amortization of regulatory assets/liability 507 1,471
Net benefit cost CAD 6,452 CAD 6,380