Shareholders' Equity - Activities Under Share Repurchase Program (Detail) - CAD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 19, 2020
Number of Common Shares repurchased		5,300,000		13,700,000
Amount of repurchase (in millions of Canadian dollars)		$ 396		$ 916
2021 Normal Course Issuer Bid (NCIB)
Number of Common Shares repurchased	0		0
Weighted-average price per share	$ 0		$ 0
Amount of repurchase (in millions of Canadian dollars)	$ 0		$ 0
2019 Normal Course Issuer Bid (NCIB)
Number of Common Shares repurchased		5,224,340		13,257,910	21,400,000
Weighted-average price per share		$ 75.88		$ 69.11
Amount of repurchase (in millions of Canadian dollars)		$ 396		$ 916	$ 1,577