Other comprehensive income (loss) information - Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at January 1	$ (677)
Balance at March 31	(661)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at January 1	(677)	$ (512)
Current period change excluding amounts reclassified from accumulated other comprehensive income	4	21
Amounts reclassified from accumulated other comprehensive income	12	10
Balance at March 31	$ (661)	$ (481)