Common Share Activities (Detail) - CAD ($) shares in Thousands, $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock beginning balance, shares		782,565
Common stock, ending balance, shares		743,902	782,565
Common stock beginning balance, value	[1]	$ 1,446
Common stock, ending balance, value	[1]	$ 1,375	$ 1,446
Common Stock
Class of Stock [Line Items]
Common stock beginning balance, shares		782,565	831,242	847,599
Issued under employee share-based awards, shares		1	2	2
Purchases at stated value, shares		(38,664)	(48,679)	(16,359)
Common stock, ending balance, shares		743,902	782,565	831,242
Common stock beginning balance, value		$ 1,446	$ 1,536	$ 1,566
Issued under employee share-based awards, value		0	0	0
Purchases at stated value, value		(71)	(90)	(30)
Common stock, ending balance, value		$ 1,375	$ 1,446	$ 1,536

[1] Number of common shares authorized and outstanding were 1,100 million and 744 million, respectively (2018 – 1,100 million and 783 million, respectively), (note 11).