Shareholders' Equity - Activities Under Share Repurchase Program (Detail) - CAD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 19, 2020
Number of Common Shares repurchased		1,600,000
Amount of repurchase (in millions of Canadian dollars)		$ 468
2021 Normal Course Issuer Bid (NCIB)
Number of Common Shares repurchased	0
Weighted-average price per share	$ 0
Amount of repurchase (in millions of Canadian dollars)	$ 0
2019 Normal Course Issuer Bid (NCIB)
Number of Common Shares repurchased		1,455,854	4,270,000
Weighted-average price per share		$ 321.71
Amount of repurchase (in millions of Canadian dollars)		$ 468	$ 1,577