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Provisions - Summary of Changes to Decommissioning Liability (Detail) - CAD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Disclosure of Provisions [line items]    
Balance, beginning of year $ 100 $ 129
Net liabilities added (disposed) 0 (6)
Increase (decrease) due to changes in estimates (42) (10)
Liabilities settled (9) (14)
Transfers from (to) liabilities for assets held for sale 7 7
Accretion charges 4 8
Balance, end of year 60 100
Current portion 8 13
Long-term portion $ 52 $ 87