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Schedule of Asset Retirement Obligations (Detail) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Reconciliation of Changes in Asset Retirement Obligations [Line Items]    
Balance as at January 1 $ 1,417 $ 1,397
Additions (deductions) (23) (5)
Accretion 80 85
Settlement (74) (60)
Balance as at December 31 $ 1,400 $ 1,417