Common Share Activities (Detail) - CAD ($) shares in Thousands, $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Common stock beginning balance, shares		743,902
Common stock, ending balance, shares		734,077	743,902
Common stock beginning balance, value	[1]	$ 1,375
Common stock, ending balance, value	[1]	$ 1,357	$ 1,375
Common Stock
Class of Stock [Line Items]
Common stock beginning balance, shares		743,902	782,565	831,242
Issued under employee share-based awards, shares		7	1	2
Purchases at stated value, shares		(9,832)	(38,664)	(48,679)
Common stock, ending balance, shares		734,077	743,902	782,565
Common stock beginning balance, value		$ 1,375	$ 1,446	$ 1,536
Issued under employee share-based awards, value			0	0
Purchases at stated value, value		(18)	(71)	(90)
Common stock, ending balance, value		$ 1,357	$ 1,375	$ 1,446

[1] Number of common shares authorized and outstanding were 1,100 million and 734 million, respectively (2019 – 1,100 million and 744 million, respectively), (note 11).