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Derivative and Hedging Instruments - Net Derivative Significant Unobservable Inputs Used in Fair Value Measurement (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of Roll forwards for net derivative contracts measured at fair value using significant unobservable inputs abstract    
Balance at the beginning of the year $ 769 $ 163
Net income (666) 1,082
OCI (48) (9)
Purchases 12 22
Settlements 18 (103)
Into Level 3 9  
Out of Level 3 (13) (363)
Currency movement 25 (23)
Balance at the end of the year 106 769
Change in unrealized gains (losses) on instruments still held $ (460) $ 832