Shareholders' Equity - Activities Under Share Repurchase Program (Detail) - CAD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Number of Common Shares repurchased	1,000,000.0	1,600,000	2,700,000		3,200,000
Amount of repurchase (in millions of Canadian dollars)	$ 396	$ 471	$ 916		$ 952
Current Normal Course Issuer Bid (NCIB)
Number of Common Shares repurchased	1,044,868		2,651,582	2,950,000
Weighted-average price per share	$ 379.42		$ 345.56
Amount of repurchase (in millions of Canadian dollars)	$ 396		$ 916	$ 1,016
2018 Normal Course Issuer Bid (NCIB)
Number of Common Shares repurchased		1,519,540			3,183,461
Weighted-average price per share		$ 310.36			$ 299.09
Amount of repurchase (in millions of Canadian dollars)		$ 471			$ 952