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Share Capital and Earnings Per share - Additional Information (Detail) - CAD ($)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Feb. 07, 2019
Nov. 12, 2018
Dec. 31, 2018
Major ordinary share transactions [Member]      
Disclosure of Dividends [line items]      
Stock repurchased for cancellation, share     23
Stock repurchased for cancellation, value     $ 478
Major ordinary share transactions [Member] | Shareholders' Retained Earnings [Member]      
Disclosure of Dividends [line items]      
Stock repurchased for cancellation, value     $ 209
Major ordinary share transactions [Member] | Dividend Reinvestment Plan [member]      
Disclosure of Dividends [line items]      
Percentage of share purchase discount for payment of dividend     2.00%
Common shares [Member]      
Disclosure of Dividends [line items]      
Stock repurchased for cancellation, share     (23)
Stock repurchased for cancellation, value     $ 269
Common shares [Member] | Major ordinary share transactions [Member]      
Disclosure of Dividends [line items]      
Stock approved to be repurchased for cancellation, share   40  
Stock repurchased for cancellation, value     $ 269
Dividend declared but not paid $ 0.25