Common shares - Common Share Activities (Detail) - CAD ($) shares in Thousands, $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock beginning balance, (in shares)		535,837
Common stock, ending balance, (in shares)		535,837		535,837
Common stock beginning balance	[1]	$ 992
Common stock, ending balance	[1]	$ 992		$ 992
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock beginning balance, (in shares)		535,837	584,153	584,153
Purchases at stated value, (in shares)		0		(48,316)
Common stock, ending balance, (in shares)		535,837		535,837
Common stock beginning balance		$ 992	$ 1,079	$ 1,079
Purchases at stated value		0	$ 0	(87)
Common stock, ending balance		$ 992		$ 992

[1] (d) Number of common shares authorized (millions).1,1001,100Number of common shares outstanding (millions).536536