Common Share Activities (Detail) - CAD ($) shares in Thousands, $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Class of Stock [Line Items]
Common stock beginning balance, shares		734,077
Common stock, ending balance, shares		734,077		734,077
Common stock beginning balance, value	[1]	$ 1,357
Common stock, ending balance, value	[1]	$ 1,357		$ 1,357
Common Stock
Class of Stock [Line Items]
Common stock beginning balance, shares		734,077	743,902	743,902
Issued under employee share-based awards, shares		0		7
Purchases at stated value, shares		0		(9,832)
Common stock, ending balance, shares		734,077		734,077
Common stock beginning balance, value		$ 1,357	$ 1,375	$ 1,375
Issued under employee share-based awards, value		0		0
Purchases at stated value, value		0	$ (18)	(18)
Common stock, ending balance, value		$ 1,357		$ 1,357

[1] Number of common shares authorized and outstanding were 1,100 million and 734 million, respectively (2020 - 1,100 million and 734 million, respectively).